diff --git a/.github/workflows/release.yaml b/.github/workflows/release.yaml
index 9ca9bf8..da98f90 100644
--- a/.github/workflows/release.yaml
+++ b/.github/workflows/release.yaml
@@ -19,6 +19,14 @@ jobs:
           node-version: 24
           registry-url: https://registry.npmjs.org
       - run: npm ci
+      - name: The tag must match the version being published
+        run: |
+          tagged="${GITHUB_REF_NAME#v}"
+          packaged="$(node -p 'require("./package.json").version')"
+          test "$tagged" = "$packaged" || {
+            echo "tag $GITHUB_REF_NAME does not match package.json $packaged"
+            exit 1
+          }
       - run: npm run lint
       - run: npm test
       - run: npm publish --provenance --access public
diff --git a/AGENTS.md b/AGENTS.md
index 55f0337..3a30f9d 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -35,21 +35,33 @@ These are not preferences. Breaking one is a defect.
 
 ```
 src/
-	index.ts          Public surface. Named exports only, no default export.
-	client.ts         client() -> { err, session }
-	server.ts         server() -> { err, server }, server owns the listener + close()
-	session.ts        Session: framing, sequence numbers, the send window, events
-	sms.ts            The live handle emitted as the 'sms' event (sendResp/sendDlr)
-	message.ts        Encoding detection, splitting, bit counting, SMPP date formatting
-	pdu.ts            pduToObj / objToPdu / pduReturn — synchronous, result-returning
-	result.ts         Result<T> — the shape every fallible call returns
+	index.ts             Public surface. Named exports only, no default export.
+	client.ts            client() -> { err, session }
+	server.ts            server() -> { err, server }, server owns the listener + close()
+	session.ts           Session: dispatch, events, and the collaborators below
+	sms.ts               The live handle emitted as the 'sms' event (sendResp/sendDlr)
+	dlr.ts               Delivery receipts: text and TLV parsing, receipt status codes
+	dlr-merger.ts        DlrMerger: per-segment receipts counted into one MessageDlr
+	link-timers.ts       LinkTimers: the enquire_link heartbeat and the idle timeout
+	log.ts               silentLog — the default when the application passes none
+	message.ts           Encoding detection, splitting, bit counting, SMPP date formatting
+	pdu.ts               pduToObj / objToPdu / pduReturn — synchronous, result-returning
+	pdu-framer.ts        PduFramer: a byte stream cut into complete PDUs
+	pending-requests.ts  PendingRequests: sequence numbers, correlation, timeout, abort
+	reassembly.ts        Reassembler: capped, expiring multipart groups
+	reconnect-loop.ts    ReconnectLoop: backoff, retry timer, stopped-ness
+	result.ts            Result<T> — the shape every fallible call returns
+	send-sms.ts          submitSms composition and the submitSmParams builder
+	send-window.ts       SendWindow: the maxOutstanding semaphore
+	udh.ts               User data header: the concatenation fields of a long SMS
+	uuid.ts              uuidv7() — the ids the library generates for messages
 	defs/
-		commands.ts   The 33 commands, their ids and ordered parameter lists
-		constants.ts  consts + constsById (TON, NPI, ENCODING, MESSAGE_STATE, …)
-		encodings.ts  GSM 03.38, LATIN1, UCS2, detection, data_coding resolution
-		errors.ts     errors + errorsById (ESME_*)
-		tlvs.ts       TLV definitions, tlvsById
-		types.ts      Wire types: int8/int16/int32/string/cstring/buffer/arrays
+		commands.ts      The 33 commands, their ids and ordered parameter lists
+		constants.ts     consts + constsById, and the SMPP version constants
+		encodings.ts     GSM 03.38, LATIN1, UCS2, detection, data_coding resolution
+		errors.ts        errors + errorsById (ESME_*)
+		tlvs.ts          TLV definitions, tlvsById
+		types.ts         Wire types: int8/int16/int32/string/cstring/buffer/arrays
 ```
 
 Dependency direction is one way: `defs` knows nothing above it, `pdu` uses `defs`, `session` uses
@@ -142,6 +154,12 @@ exactly 140.
 
 ## Decisions
 
+- **The library speaks SMPP 3.4 on the wire, and `defs/` keeps the 5.0 tables as a superset.**
+  Maintainer's call, 2026-08-26: 3.4 is what SMSCs actually run, while the wider tables let the codec
+  parse and build whatever a peer sends. The declared version is an option on both `client()` and
+  `server()`, so an implementation that needs 5.0 throughout can have it. The threshold at or above
+  which a peer may be sent optional parameters is fixed at 0x34 by the spec and is not the same
+  constant as the declared version.
 - **The TLS tests build their own self-signed certificate in DER** (`test/tls.test.ts`) instead of
   adding a devDependency or shelling out to openssl. Maintainer's call, 2026-08-26: the dev image
   `node:24.18.0-bookworm-slim` ships no openssl binary, so a shelled-out fixture would pass in CI
diff --git a/README.md b/README.md
index fc99c1f..4cc2cf2 100644
--- a/README.md
+++ b/README.md
@@ -166,6 +166,8 @@ await smpp.close();      // stop listening and close every live session
 | --- | --- | --- |
 | `host` / `port` | all interfaces / `2775` | Where to listen. Pass `0` for any free port. |
 | `authenticate` | accept everything | `({ password, session, systemId, systemType }) => false \| { userData }`, sync or async. |
+| `systemId` | `''` | The SMSC identity returned to the ESME in the bind response. |
+| `interfaceVersion` | `0x34` | The SMPP version advertised in the bind response. The floor for sending a peer optional parameters stays `0x34`, whatever this is set to. |
 | `tls` | `false` | A `tls.TlsOptions` object with your certificate and key. |
 | `idleTimeout` | `40000` | Drop a peer that has been silent this long. |
 | `maxReassembly` | `1000` | Incomplete multipart messages held per session. |
@@ -275,6 +277,10 @@ have worked around any of these, remove the workaround:
 - A message whose last octet was `0x00` was allocated one octet short while `sm_length` still
   reported the full length, so it went out corrupt. In UCS2 that is any message ending in a
   character like 一 (U+4E00), which made the bug routine for CJK text.
+- Binary TLVs (`message_payload`, `network_error_code`, `callback_num` and the rest) were parsed
+  into a hex string and written back as the ASCII of that string, so every one that made a round
+  trip went out corrupt. They are `Buffer`s in both directions now, so drop any hex encoding of
+  your own.
 - Short or malformed PDUs threw out of the codec instead of being reported as a parse failure.
 - Binds now declare `interface_version` 0x34. 0.4.0 declared 0x00, which tells the SMSC the ESME
   speaks SMPP 3.3 or earlier — and a spec-following SMSC then withholds every optional parameter,
diff --git a/eslint.config.js b/eslint.config.js
index b1e44ec..0216ded 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -24,6 +24,29 @@ export default tseslint.config(
 			'no-console': 'error',
 		},
 	},
+	{
+		files: ['src/**/*.ts'],
+		rules: {
+			complexity: ['error', 10],
+			'max-lines': ['error', { max: 350, skipBlankLines: true, skipComments: true }],
+			'max-lines-per-function': ['error', { max: 40, skipBlankLines: true, skipComments: true }],
+			'max-params': ['error', 5],
+		},
+	},
+	{
+		// The spec tables are data: their length tracks the specification, not any complexity.
+		files: ['src/defs/*.ts'],
+		rules: { 'max-lines': 'off' },
+	},
+	{
+		// The codec branches per wire type and per parameter; splitting it scatters the wire format
+		// across files instead. These two keep the ceiling they have today.
+		files: ['src/dlr.ts', 'src/pdu.ts'],
+		rules: {
+			complexity: ['error', 22],
+			'max-lines-per-function': ['error', { max: 75, skipBlankLines: true, skipComments: true }],
+		},
+	},
 	{
 		// ESC (0x1B) is the GSM 03.38 escape character, so it belongs in these patterns.
 		files: ['src/defs/encodings.ts'],
diff --git a/src/client.ts b/src/client.ts
index f74b88f..9f951c1 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -5,6 +5,7 @@ import type { Socket } from 'node:net';
 import { Session } from './session.ts';
 import { connect as netConnect } from 'node:net';
 import { connect as tlsConnect } from 'node:tls';
+import { defaultInterfaceVersion } from './defs/constants.ts';
 import { silentLog } from './log.ts';
 
 export type BindType = 'receiver' | 'transceiver' | 'transmitter';
@@ -33,8 +34,7 @@ const defaults = {
 	bindType: 'transceiver',
 	enquireLinkInterval: 20_000,
 	host: 'localhost',
-	/** SMPP 3.4. The 3.4 spec reserves every value above it, so 0x50 is undefined to a 3.4 SMSC. */
-	interfaceVersion: 0x34,
+	interfaceVersion: defaultInterfaceVersion,
 	password: 'pass',
 	port: 2775,
 	username: 'user',
@@ -45,9 +45,12 @@ const defaults = {
  * handshake, so those connections were not encrypted at all.
  */
 function openSocket(options: ClientOptions): Promise<Result<{ sock: Socket }>> {
+	const host = options.host ?? defaults.host;
+	const port = options.port ?? defaults.port;
+	const secure = options.tls !== undefined && options.tls !== false;
+	const tlsOptions = typeof options.tls === 'object' ? options.tls : undefined;
+
 	return new Promise(resolve => {
-		const host = options.host ?? defaults.host;
-		const port = options.port ?? defaults.port;
 		const signal = options.signal;
 
 		if (signal?.aborted === true) {
@@ -56,13 +59,7 @@ function openSocket(options: ClientOptions): Promise<Result<{ sock: Socket }>> {
 			return;
 		}
 
-		const sock = options.tls === undefined || options.tls === false
-			? netConnect({ host, port })
-			: tlsConnect({
-				host,
-				port,
-				...(typeof options.tls === 'object' ? options.tls : {}),
-			});
+		const sock = secure ? tlsConnect({ host, port, ...tlsOptions }) : netConnect({ host, port });
 
 		const settle = (result: Result<{ sock: Socket }>): void => {
 			sock.removeListener('error', onError);
@@ -81,26 +78,30 @@ function openSocket(options: ClientOptions): Promise<Result<{ sock: Socket }>> {
 
 		sock.once('error', onError);
 		signal?.addEventListener('abort', onAbort, { once: true });
-		sock.once(options.tls === undefined || options.tls === false ? 'connect' : 'secureConnect', () => {
+		sock.once(secure ? 'secureConnect' : 'connect', () => {
 			settle({ sock });
 		});
 	});
 }
 
+function bindParams(options: ClientOptions, systemId: string) {
+	return {
+		address_range: options.addressRange ?? '',
+		addr_npi: options.addrNpi ?? 0,
+		addr_ton: options.addrTon ?? 0,
+		interface_version: options.interfaceVersion ?? defaults.interfaceVersion,
+		password: options.password ?? defaults.password,
+		system_id: systemId,
+		system_type: options.systemType ?? '',
+	};
+}
+
 async function bind(session: Session, options: ClientOptions): Promise<VoidResult> {
 	const bindType = options.bindType ?? defaults.bindType;
 	const systemId = options.username ?? defaults.username;
 	const sent = await session.send({
 		cmdName: `bind_${bindType}`,
-		params: {
-			address_range: options.addressRange ?? '',
-			addr_npi: options.addrNpi ?? 0,
-			addr_ton: options.addrTon ?? 0,
-			interface_version: options.interfaceVersion ?? defaults.interfaceVersion,
-			password: options.password ?? defaults.password,
-			system_id: systemId,
-			system_type: options.systemType ?? '',
-		},
+		params: bindParams(options, systemId),
 		...(options.signal ? { signal: options.signal } : {}),
 	});
 
diff --git a/src/defs/constants.ts b/src/defs/constants.ts
index 5b7f2f6..8980057 100644
--- a/src/defs/constants.ts
+++ b/src/defs/constants.ts
@@ -1,3 +1,9 @@
+/** The version declared on the wire. The tables below cover 5.0, which is a superset of it. */
+export const defaultInterfaceVersion = 0x34;
+
+/** Spec rule, not a preference: a peer declaring less than 3.4 is sent no optional parameters. */
+export const optionalParamsMinVersion = 0x34;
+
 export const consts = {
 	BROADCAST_AREA_FORMAT: {
 		ALIAS: 0x00,
diff --git a/src/defs/tlvs.ts b/src/defs/tlvs.ts
index 39670a3..10c52e6 100644
--- a/src/defs/tlvs.ts
+++ b/src/defs/tlvs.ts
@@ -8,96 +8,92 @@ export type TlvDefinition = {
 	type: WireType;
 };
 
-type TlvSpec = { id: number; multiple?: boolean; type: WireType };
+/** The constraint keys every definition to its own name, so a `tag` that drifts fails to compile. */
+const tlvSpecs = <T extends { [K in keyof T]: { id: number; multiple?: boolean; tag: K; type: WireType } }>(
+	definitions: T,
+): T => definitions;
 
 // Ordered by tag id, mirroring the SMPP 5.0 TLV table.
-const specs = {
-	dest_addr_subunit: { id: 0x0005, type: tlv.int8 },
-	dest_network_type: { id: 0x0006, type: tlv.int8 },
-	dest_bearer_type: { id: 0x0007, type: tlv.int8 },
-	dest_telematics_id: { id: 0x0008, type: tlv.int16 },
-	source_addr_subunit: { id: 0x000D, type: tlv.int8 },
-	source_network_type: { id: 0x000E, type: tlv.int8 },
-	source_bearer_type: { id: 0x000F, type: tlv.int8 },
-	source_telematics_id: { id: 0x0010, type: tlv.int16 },
-	qos_time_to_live: { id: 0x0017, type: tlv.int32 },
-	payload_type: { id: 0x0019, type: tlv.int8 },
-	additional_status_info_text: { id: 0x001D, type: tlv.cstring },
-	receipted_message_id: { id: 0x001E, type: tlv.cstring },
-	ms_msg_wait_facilities: { id: 0x0030, type: tlv.int8 },
-	privacy_indicator: { id: 0x0201, type: tlv.int8 },
-	source_subaddress: { id: 0x0202, type: tlv.buffer },
-	dest_subaddress: { id: 0x0203, type: tlv.buffer },
-	user_message_reference: { id: 0x0204, type: tlv.int16 },
-	user_response_code: { id: 0x0205, type: tlv.int8 },
-	source_port: { id: 0x020A, type: tlv.int16 },
-	dest_port: { id: 0x020B, type: tlv.int16 },
-	sar_msg_ref_num: { id: 0x020C, type: tlv.int16 },
-	language_indicator: { id: 0x020D, type: tlv.int8 },
-	sar_total_segments: { id: 0x020E, type: tlv.int8 },
-	sar_segment_seqnum: { id: 0x020F, type: tlv.int8 },
-	sc_interface_version: { id: 0x0210, type: tlv.int8 },
-	callback_num_pres_ind: { id: 0x0302, multiple: true, type: tlv.int8 },
-	callback_num_atag: { id: 0x0303, multiple: true, type: tlv.buffer },
-	number_of_messages: { id: 0x0304, type: tlv.int8 },
-	callback_num: { id: 0x0381, multiple: true, type: tlv.buffer },
-	dpf_result: { id: 0x0420, type: tlv.int8 },
-	set_dpf: { id: 0x0421, type: tlv.int8 },
-	ms_availability_status: { id: 0x0422, type: tlv.int8 },
-	network_error_code: { id: 0x0423, type: tlv.buffer },
-	message_payload: { id: 0x0424, type: tlv.buffer },
-	delivery_failure_reason: { id: 0x0425, type: tlv.int8 },
-	more_messages_to_send: { id: 0x0426, type: tlv.int8 },
-	message_state: { id: 0x0427, type: tlv.int8 },
-	congestion_state: { id: 0x0428, type: tlv.int8 },
-	ussd_service_op: { id: 0x0501, type: tlv.int8 },
-	broadcast_channel_indicator: { id: 0x0600, type: tlv.int8 },
-	broadcast_content_type: { id: 0x0601, type: tlv.buffer },
-	broadcast_content_type_info: { id: 0x0602, type: tlv.string },
-	broadcast_message_class: { id: 0x0603, type: tlv.int8 },
-	broadcast_rep_num: { id: 0x0604, type: tlv.int16 },
-	broadcast_frequency_interval: { id: 0x0605, type: tlv.buffer },
-	broadcast_area_identifier: { id: 0x0606, multiple: true, type: tlv.buffer },
-	broadcast_error_status: { id: 0x0607, multiple: true, type: tlv.int32 },
-	broadcast_area_success: { id: 0x0608, type: tlv.int8 },
-	broadcast_end_time: { id: 0x0609, type: tlv.string },
-	broadcast_service_group: { id: 0x060A, type: tlv.string },
-	billing_identification: { id: 0x060B, type: tlv.buffer },
-	source_network_id: { id: 0x060D, type: tlv.cstring },
-	dest_network_id: { id: 0x060E, type: tlv.cstring },
-	source_node_id: { id: 0x060F, type: tlv.string },
-	dest_node_id: { id: 0x0610, type: tlv.string },
-	dest_addr_np_resolution: { id: 0x0611, type: tlv.int8 },
-	dest_addr_np_information: { id: 0x0612, type: tlv.string },
-	dest_addr_np_country: { id: 0x0613, type: tlv.int32 },
-	display_time: { id: 0x1201, type: tlv.int8 },
-	sms_signal: { id: 0x1203, type: tlv.int16 },
-	ms_validity: { id: 0x1204, type: tlv.buffer },
-	alert_on_message_delivery: { id: 0x130C, type: tlv.int8 },
-	its_reply_type: { id: 0x1380, type: tlv.int8 },
-	its_session_info: { id: 0x1383, type: tlv.buffer },
-} satisfies Record<string, TlvSpec>;
+const specs = tlvSpecs({
+	dest_addr_subunit: { id: 0x0005, tag: 'dest_addr_subunit', type: tlv.int8 },
+	dest_network_type: { id: 0x0006, tag: 'dest_network_type', type: tlv.int8 },
+	dest_bearer_type: { id: 0x0007, tag: 'dest_bearer_type', type: tlv.int8 },
+	dest_telematics_id: { id: 0x0008, tag: 'dest_telematics_id', type: tlv.int16 },
+	source_addr_subunit: { id: 0x000D, tag: 'source_addr_subunit', type: tlv.int8 },
+	source_network_type: { id: 0x000E, tag: 'source_network_type', type: tlv.int8 },
+	source_bearer_type: { id: 0x000F, tag: 'source_bearer_type', type: tlv.int8 },
+	source_telematics_id: { id: 0x0010, tag: 'source_telematics_id', type: tlv.int16 },
+	qos_time_to_live: { id: 0x0017, tag: 'qos_time_to_live', type: tlv.int32 },
+	payload_type: { id: 0x0019, tag: 'payload_type', type: tlv.int8 },
+	additional_status_info_text: { id: 0x001D, tag: 'additional_status_info_text', type: tlv.cstring },
+	receipted_message_id: { id: 0x001E, tag: 'receipted_message_id', type: tlv.cstring },
+	ms_msg_wait_facilities: { id: 0x0030, tag: 'ms_msg_wait_facilities', type: tlv.int8 },
+	privacy_indicator: { id: 0x0201, tag: 'privacy_indicator', type: tlv.int8 },
+	source_subaddress: { id: 0x0202, tag: 'source_subaddress', type: tlv.buffer },
+	dest_subaddress: { id: 0x0203, tag: 'dest_subaddress', type: tlv.buffer },
+	user_message_reference: { id: 0x0204, tag: 'user_message_reference', type: tlv.int16 },
+	user_response_code: { id: 0x0205, tag: 'user_response_code', type: tlv.int8 },
+	source_port: { id: 0x020A, tag: 'source_port', type: tlv.int16 },
+	dest_port: { id: 0x020B, tag: 'dest_port', type: tlv.int16 },
+	sar_msg_ref_num: { id: 0x020C, tag: 'sar_msg_ref_num', type: tlv.int16 },
+	language_indicator: { id: 0x020D, tag: 'language_indicator', type: tlv.int8 },
+	sar_total_segments: { id: 0x020E, tag: 'sar_total_segments', type: tlv.int8 },
+	sar_segment_seqnum: { id: 0x020F, tag: 'sar_segment_seqnum', type: tlv.int8 },
+	sc_interface_version: { id: 0x0210, tag: 'sc_interface_version', type: tlv.int8 },
+	callback_num_pres_ind: { id: 0x0302, multiple: true, tag: 'callback_num_pres_ind', type: tlv.int8 },
+	callback_num_atag: { id: 0x0303, multiple: true, tag: 'callback_num_atag', type: tlv.buffer },
+	number_of_messages: { id: 0x0304, tag: 'number_of_messages', type: tlv.int8 },
+	callback_num: { id: 0x0381, multiple: true, tag: 'callback_num', type: tlv.buffer },
+	dpf_result: { id: 0x0420, tag: 'dpf_result', type: tlv.int8 },
+	set_dpf: { id: 0x0421, tag: 'set_dpf', type: tlv.int8 },
+	ms_availability_status: { id: 0x0422, tag: 'ms_availability_status', type: tlv.int8 },
+	network_error_code: { id: 0x0423, tag: 'network_error_code', type: tlv.buffer },
+	message_payload: { id: 0x0424, tag: 'message_payload', type: tlv.buffer },
+	delivery_failure_reason: { id: 0x0425, tag: 'delivery_failure_reason', type: tlv.int8 },
+	more_messages_to_send: { id: 0x0426, tag: 'more_messages_to_send', type: tlv.int8 },
+	message_state: { id: 0x0427, tag: 'message_state', type: tlv.int8 },
+	congestion_state: { id: 0x0428, tag: 'congestion_state', type: tlv.int8 },
+	ussd_service_op: { id: 0x0501, tag: 'ussd_service_op', type: tlv.int8 },
+	broadcast_channel_indicator: { id: 0x0600, tag: 'broadcast_channel_indicator', type: tlv.int8 },
+	broadcast_content_type: { id: 0x0601, tag: 'broadcast_content_type', type: tlv.buffer },
+	broadcast_content_type_info: { id: 0x0602, tag: 'broadcast_content_type_info', type: tlv.string },
+	broadcast_message_class: { id: 0x0603, tag: 'broadcast_message_class', type: tlv.int8 },
+	broadcast_rep_num: { id: 0x0604, tag: 'broadcast_rep_num', type: tlv.int16 },
+	broadcast_frequency_interval: { id: 0x0605, tag: 'broadcast_frequency_interval', type: tlv.buffer },
+	broadcast_area_identifier: { id: 0x0606, multiple: true, tag: 'broadcast_area_identifier', type: tlv.buffer },
+	broadcast_error_status: { id: 0x0607, multiple: true, tag: 'broadcast_error_status', type: tlv.int32 },
+	broadcast_area_success: { id: 0x0608, tag: 'broadcast_area_success', type: tlv.int8 },
+	broadcast_end_time: { id: 0x0609, tag: 'broadcast_end_time', type: tlv.string },
+	broadcast_service_group: { id: 0x060A, tag: 'broadcast_service_group', type: tlv.string },
+	billing_identification: { id: 0x060B, tag: 'billing_identification', type: tlv.buffer },
+	source_network_id: { id: 0x060D, tag: 'source_network_id', type: tlv.cstring },
+	dest_network_id: { id: 0x060E, tag: 'dest_network_id', type: tlv.cstring },
+	source_node_id: { id: 0x060F, tag: 'source_node_id', type: tlv.string },
+	dest_node_id: { id: 0x0610, tag: 'dest_node_id', type: tlv.string },
+	dest_addr_np_resolution: { id: 0x0611, tag: 'dest_addr_np_resolution', type: tlv.int8 },
+	dest_addr_np_information: { id: 0x0612, tag: 'dest_addr_np_information', type: tlv.string },
+	dest_addr_np_country: { id: 0x0613, tag: 'dest_addr_np_country', type: tlv.int32 },
+	display_time: { id: 0x1201, tag: 'display_time', type: tlv.int8 },
+	sms_signal: { id: 0x1203, tag: 'sms_signal', type: tlv.int16 },
+	ms_validity: { id: 0x1204, tag: 'ms_validity', type: tlv.buffer },
+	alert_on_message_delivery: { id: 0x130C, tag: 'alert_on_message_delivery', type: tlv.int8 },
+	its_reply_type: { id: 0x1380, tag: 'its_reply_type', type: tlv.int8 },
+	its_session_info: { id: 0x1383, tag: 'its_session_info', type: tlv.buffer },
+});
 
 export type TlvName = keyof typeof specs;
 
-export const tlvs: Record<string, TlvDefinition> = {};
-export const tlvsById: Record<number, TlvDefinition> = {};
-
-for (const [tag, spec] of Object.entries<TlvSpec>(specs)) {
-	const definition: TlvDefinition = { ...spec, tag };
-
-	tlvs[tag] = definition;
-	tlvsById[spec.id] = definition;
-}
-
-// Alternate spellings that resolve to the same tag; the definition keeps its canonical name.
-const aliases: Record<string, TlvName> = {
-	alert_on_msg_delivery: 'alert_on_message_delivery',
-	failed_broadcast_area_identifier: 'broadcast_area_identifier',
+export const tlvs: Record<TlvName, TlvDefinition> & Record<string, TlvDefinition> = {
+	...specs,
+	// Alternate spellings; the definition behind each keeps its canonical name.
+	alert_on_msg_delivery: specs.alert_on_message_delivery,
+	failed_broadcast_area_identifier: specs.broadcast_area_identifier,
 };
 
-for (const [alias, target] of Object.entries(aliases)) {
-	tlvs[alias] = { ...specs[target], tag: target };
+export const tlvsById: Record<number, TlvDefinition> = {};
+
+for (const definition of Object.values<TlvDefinition>(specs)) {
+	tlvsById[definition.id] = definition;
 }
 
 /** Fallback for tags this table does not know: keep the raw octets. */
diff --git a/src/dlr-merger.ts b/src/dlr-merger.ts
new file mode 100644
index 0000000..2ed09ab
--- /dev/null
+++ b/src/dlr-merger.ts
@@ -0,0 +1,64 @@
+import type { Dlr } from './dlr.ts';
+
+export type MessageDlr = Dlr & { segments: Dlr[] };
+
+type Group = {
+	expected: number;
+	parts: Map<number, Dlr>;
+};
+
+const numbered = /^(.*)-(\d+)$/;
+
+/**
+ * Merges the per-segment receipts of a multipart message into one report, but only when the peer
+ * numbered its ids `<base>-<n>` off one base — the convention this library's own server follows. An
+ * SMSC that hands out unrelated ids per segment cannot be merged, so nothing is reported for it.
+ */
+export class DlrMerger {
+	private readonly groups = new Map<string, Group>();
+
+	/** Registers the ids one multipart send got back, so their receipts can be merged. */
+	expect(smsIds: string[]): void {
+		if (smsIds.length < 2) return;
+
+		const bases = new Set<string>();
+
+		for (const smsId of smsIds) {
+			const base = numbered.exec(smsId)?.[1];
+
+			if (base === undefined || base === '') return;
+
+			bases.add(base);
+		}
+
+		if (bases.size !== 1) return;
+
+		for (const base of bases) {
+			this.groups.set(base, { expected: smsIds.length, parts: new Map() });
+		}
+	}
+
+	/** The whole message's report, on the receipt that completes it. */
+	collect(dlr: Dlr): MessageDlr | undefined {
+		const match = numbered.exec(dlr.smsId);
+		const base = match?.[1];
+		const part = match?.[2];
+
+		if (base === undefined || part === undefined) return undefined;
+
+		const group = this.groups.get(base);
+
+		if (!group) return undefined;
+
+		group.parts.set(Number(part), dlr);
+
+		if (group.parts.size < group.expected) return undefined;
+
+		this.groups.delete(base);
+
+		const segments = [...group.parts.entries()].sort(([a], [b]) => a - b).map(([, one]) => one);
+		const worst = segments.reduce((carry, one) => (one.statusId > carry.statusId ? one : carry));
+
+		return { ...worst, segments, smsId: base };
+	}
+}
diff --git a/src/link-timers.ts b/src/link-timers.ts
new file mode 100644
index 0000000..0fed181
--- /dev/null
+++ b/src/link-timers.ts
@@ -0,0 +1,50 @@
+import type { LogInt } from '@larvit/log';
+
+export type LinkTimersOptions = {
+	/** How long between enquire_link probes. Undefined or 0 never probes. */
+	enquireLinkInterval?: number | undefined;
+	/** How long a silent peer is kept. Undefined or 0 keeps it forever. */
+	idleTimeout?: number | undefined;
+	log: LogInt;
+	onEnquireLink: () => void;
+	onIdle: () => void;
+};
+
+/** Keeps a quiet connection honest: probes the peer, and gives up on one that stays silent. */
+export class LinkTimers {
+	private readonly options: LinkTimersOptions;
+	private enquireLink: NodeJS.Timeout | undefined;
+	private idle: NodeJS.Timeout | undefined;
+
+	constructor(options: LinkTimersOptions) {
+		this.options = options;
+	}
+
+	/** Starts both timers over, which every sign of life from the peer should do. */
+	reset(): void {
+		const { enquireLinkInterval, idleTimeout, log, onEnquireLink, onIdle } = this.options;
+
+		this.clear();
+
+		if (enquireLinkInterval !== undefined && enquireLinkInterval > 0) {
+			this.enquireLink = setTimeout(onEnquireLink, enquireLinkInterval);
+			this.enquireLink.unref();
+		}
+
+		if (idleTimeout !== undefined && idleTimeout > 0) {
+			this.idle = setTimeout(() => {
+				log.info('linkTimers - closing an idle peer', { idleTimeout });
+				onIdle();
+			}, idleTimeout);
+			this.idle.unref();
+		}
+	}
+
+	clear(): void {
+		if (this.enquireLink) clearTimeout(this.enquireLink);
+		if (this.idle) clearTimeout(this.idle);
+
+		this.enquireLink = undefined;
+		this.idle = undefined;
+	}
+}
diff --git a/src/pdu.ts b/src/pdu.ts
index 29e49da..1ead29a 100644
--- a/src/pdu.ts
+++ b/src/pdu.ts
@@ -8,7 +8,7 @@ import { consts } from './defs/constants.ts';
 import { decodeMessage, encodeMessage } from './message.ts';
 import { detect, encodingByDataCoding } from './defs/encodings.ts';
 import { errorNameById, errors, isErrorName } from './defs/errors.ts';
-import { tlvDefault, tlvsById } from './defs/tlvs.ts';
+import { tlvDefault, tlvs, tlvsById } from './defs/tlvs.ts';
 
 /** Sequence numbers are a 31-bit field; 0x7fffffff is reserved. */
 export const maxSeqNr = 2147483646;
@@ -17,8 +17,8 @@ export const maxSeqNr = 2147483646;
 export const maxPduLength = 1024 * 1024;
 
 export type TlvInput = {
-	tagId: number;
-	tagName?: string | undefined;
+	/** Resolved from the record key; pass it for a tag the TLV table does not define. */
+	tagId?: number | undefined;
 	tagValue: ParamValue;
 };
 
@@ -65,6 +65,20 @@ function numberOr(value: ParamValue | undefined, fallback: number): number {
 	return typeof value === 'number' ? value : fallback;
 }
 
+function tagIdOf(name: string, input: TlvInput): Result<{ tagId: number }> {
+	const tagId = input.tagId ?? tlvs[name]?.id;
+
+	if (tagId === undefined) {
+		return { err: new Error(`TLV "${name}": unknown tag name, give it a tagId`) };
+	}
+
+	if (!Number.isInteger(tagId) || tagId < 0 || tagId > 0xFFFF) {
+		return { err: new Error(`TLV "${name}": tagId ${String(tagId)} out of range 0-65535`) };
+	}
+
+	return { tagId };
+}
+
 function buildPdu(
 	cmdName: CommandName,
 	cmdStatus: ErrorName,
@@ -126,16 +140,24 @@ function buildPdu(
 	}
 
 	for (const [name, tlv] of Object.entries(tlvs ?? {})) {
-		const type = tlvsById[tlv.tagId]?.type ?? tlvDefault;
+		const tag = tagIdOf(name, tlv);
+
+		if (tag.err) return { err: tag.err };
+
+		const type = tlvsById[tag.tagId]?.type ?? tlvDefault;
 		const sized = type.size(tlv.tagValue);
 
 		if (sized.err) {
 			return { err: new Error(`TLV "${name}": ${sized.err.message}`) };
 		}
 
+		if (sized.size > 0xffff) {
+			return { err: new Error(`TLV "${name}": ${String(sized.size)} octets overflow the two octet length`) };
+		}
+
 		const chunk = Buffer.alloc(sized.size + 4);
 
-		chunk.writeUInt16BE(tlv.tagId, 0);
+		chunk.writeUInt16BE(tag.tagId, 0);
 		chunk.writeUInt16BE(sized.size, 2);
 
 		const written = type.write(tlv.tagValue, chunk, 4);
@@ -192,7 +214,7 @@ function parseTlvs(
 		tlvs[definition?.tag ?? tagId.toString()] = {
 			tagId,
 			tagName: definition?.tag,
-			tagValue: Buffer.isBuffer(read.value) ? read.value.toString('hex') : read.value,
+			tagValue: read.value,
 		};
 
 		offset += 4 + tagLength;
diff --git a/src/pending-requests.ts b/src/pending-requests.ts
new file mode 100644
index 0000000..9947198
--- /dev/null
+++ b/src/pending-requests.ts
@@ -0,0 +1,88 @@
+import type { LogInt } from '@larvit/log';
+import type { PduObject } from './pdu.ts';
+import type { Result } from './result.ts';
+import { maxSeqNr } from './pdu.ts';
+
+export type WaitOptions = {
+	signal?: AbortSignal | undefined;
+	timeout: number;
+};
+
+type Pending = {
+	settle: (result: Result<{ pduObj: PduObject }>) => void;
+};
+
+/** Hands out sequence numbers and matches responses to the requests waiting for them. */
+export class PendingRequests {
+	private readonly log: LogInt;
+	private readonly pending = new Map<number, Pending>();
+	private ourSeqNr = 1;
+
+	constructor(log: LogInt) {
+		this.log = log;
+	}
+
+	nextSeqNr(): number {
+		const seqNr = this.ourSeqNr;
+
+		this.ourSeqNr = this.ourSeqNr >= maxSeqNr ? 1 : this.ourSeqNr + 1;
+
+		return seqNr;
+	}
+
+	wait(seqNr: number, options: WaitOptions): Promise<Result<{ pduObj: PduObject }>> {
+		const { signal, timeout } = options;
+
+		return new Promise(resolve => {
+			const abort = (): void => {
+				this.settle(seqNr, { err: new Error('Aborted before a response arrived') });
+			};
+			const timer = timeout > 0 ? this.expire(seqNr, timeout) : undefined;
+
+			this.pending.set(seqNr, {
+				settle: result => {
+					if (timer) clearTimeout(timer);
+
+					signal?.removeEventListener('abort', abort);
+					this.pending.delete(seqNr);
+					resolve(result);
+				},
+			});
+
+			if (signal?.aborted === true) abort();
+			else signal?.addEventListener('abort', abort, { once: true });
+		});
+	}
+
+	/** Hands a response to whoever is waiting for it. False means nobody was. */
+	deliver(pduObj: PduObject): boolean {
+		const pending = this.pending.get(pduObj.seqNr);
+
+		if (!pending) return false;
+
+		pending.settle({ pduObj });
+
+		return true;
+	}
+
+	settle(seqNr: number, result: Result<{ pduObj: PduObject }>): void {
+		this.pending.get(seqNr)?.settle(result);
+	}
+
+	settleAll(err: Error): void {
+		for (const [seqNr] of this.pending) {
+			this.settle(seqNr, { err });
+		}
+	}
+
+	private expire(seqNr: number, timeout: number): NodeJS.Timeout {
+		const timer = setTimeout(() => {
+			this.log.warn('pendingRequests - no response before the timeout', { seqNr, timeout });
+			this.settle(seqNr, { err: new Error(`No response to seqNr ${String(seqNr)}`) });
+		}, timeout);
+
+		timer.unref();
+
+		return timer;
+	}
+}
diff --git a/src/reassembly.ts b/src/reassembly.ts
new file mode 100644
index 0000000..c472591
--- /dev/null
+++ b/src/reassembly.ts
@@ -0,0 +1,139 @@
+import type { ConcatInfo } from './udh.ts';
+import type { LogInt } from '@larvit/log';
+import type { ParamValue } from './defs/types.ts';
+import type { PduObject } from './pdu.ts';
+import { decodeMessage } from './message.ts';
+import { paramText } from './defs/types.ts';
+
+export type ReassemblerOptions = {
+	log: LogInt;
+	max: number;
+	/** Injected so expiry can be exercised without a wall clock. */
+	now?: (() => number) | undefined;
+	timeout: number;
+};
+
+type Group = {
+	deadline: number;
+	parts: Map<number, PduObject>;
+	total: number;
+};
+
+function groupKey(pduObj: PduObject, reference: number): string {
+	return [
+		paramText(pduObj.params.source_addr),
+		paramText(pduObj.params.destination_addr),
+		String(reference),
+	].join('_');
+}
+
+function numberOr(value: ParamValue | undefined, fallback: number): number {
+	return typeof value === 'number' ? value : fallback;
+}
+
+/** The text of a message, joining its segments in the order they were reassembled. */
+export function decodeSegments(pduObjs: PduObject[]): string {
+	let message = '';
+
+	for (const pduObj of pduObjs) {
+		const part = pduObj.params.short_message;
+
+		message += Buffer.isBuffer(part)
+			? decodeMessage(
+				part,
+				numberOr(pduObj.params.data_coding, 0),
+				numberOr(pduObj.params.esm_class, 0),
+			).message
+			: paramText(part);
+	}
+
+	return message;
+}
+
+/** Holds the segments of incomplete multipart messages until they are whole, capped and expiring. */
+export class Reassembler {
+	private readonly groups = new Map<string, Group>();
+	private readonly log: LogInt;
+	private readonly max: number;
+	private readonly now: () => number;
+	private readonly timeout: number;
+	private sweeper: NodeJS.Timeout | undefined;
+
+	constructor(options: ReassemblerOptions) {
+		this.log = options.log;
+		this.max = options.max;
+		this.now = options.now ?? Date.now;
+		this.timeout = options.timeout;
+	}
+
+	get size(): number {
+		return this.groups.size;
+	}
+
+	/** Every segment in order, on the one that completes the message; nothing while it is short. */
+	collect(pduObj: PduObject, concat: ConcatInfo): PduObject[] | undefined {
+		this.sweep();
+
+		const key = groupKey(pduObj, concat.reference);
+		const group = this.groups.get(key) ?? this.open(key, concat.total);
+
+		group.parts.set(concat.part, pduObj);
+
+		if (group.parts.size < group.total) return undefined;
+
+		this.groups.delete(key);
+		this.idle();
+
+		return [...group.parts.entries()].sort(([a], [b]) => a - b).map(([, part]) => part);
+	}
+
+	clear(): void {
+		this.groups.clear();
+		this.idle();
+	}
+
+	/** Drops every group past its deadline. Runs before each collect and on its own timer. */
+	sweep(): void {
+		const now = this.now();
+
+		for (const [key, group] of this.groups) {
+			if (group.deadline > now) continue;
+
+			this.log.info('reassembler - incomplete message expired', { key, total: group.total });
+			this.groups.delete(key);
+		}
+
+		this.idle();
+	}
+
+	private open(key: string, total: number): Group {
+		if (this.groups.size >= this.max) this.dropOldest();
+
+		const group: Group = { deadline: this.now() + this.timeout, parts: new Map(), total };
+
+		this.groups.set(key, group);
+
+		if (!this.sweeper) {
+			this.sweeper = setInterval(() => { this.sweep(); }, this.timeout);
+			this.sweeper.unref();
+		}
+
+		return group;
+	}
+
+	private dropOldest(): void {
+		const oldest = this.groups.keys().next();
+
+		if (oldest.done) return;
+
+		this.log.warn('reassembler - buffer full, dropping the oldest message', { max: this.max });
+		this.groups.delete(oldest.value);
+	}
+
+	private idle(): void {
+		if (!this.sweeper || this.groups.size > 0) return;
+
+		clearInterval(this.sweeper);
+		this.sweeper = undefined;
+	}
+}
diff --git a/src/reconnect-loop.ts b/src/reconnect-loop.ts
new file mode 100644
index 0000000..8d63c1c
--- /dev/null
+++ b/src/reconnect-loop.ts
@@ -0,0 +1,86 @@
+import type { LogInt } from '@larvit/log';
+import type { Result, VoidResult } from './result.ts';
+import type { Socket } from 'node:net';
+
+export type ReconnectLoopOptions = {
+	connect: () => Promise<Result<{ sock: Socket }>>;
+	log: LogInt;
+	maxDelay: number;
+	minDelay: number;
+	/** Brings the owner back up on a freshly opened socket. An err means try again. */
+	onConnected: (sock: Socket) => Promise<VoidResult>;
+};
+
+/** Reopens a dropped connection, backing off between attempts until it is told to stop. */
+export class ReconnectLoop {
+	private readonly options: ReconnectLoopOptions;
+	private delay: number;
+	private halted = false;
+	private timer: NodeJS.Timeout | undefined;
+
+	constructor(options: ReconnectLoopOptions) {
+		this.options = options;
+		this.delay = options.minDelay;
+	}
+
+	/** Read through a method: stop() can land while an attempt is awaiting. */
+	isStopped(): boolean {
+		return this.halted;
+	}
+
+	schedule(): void {
+		if (this.timer || this.isStopped()) return;
+
+		const delay = this.delay;
+
+		this.options.log.info('reconnect - retrying after a drop', { delay });
+
+		this.timer = setTimeout(() => {
+			this.timer = undefined;
+			void this.run();
+		}, delay);
+		this.timer.unref();
+
+		this.delay = Math.min(delay * 2, this.options.maxDelay);
+	}
+
+	stop(): void {
+		this.halted = true;
+
+		if (this.timer) clearTimeout(this.timer);
+
+		this.timer = undefined;
+	}
+
+	private async run(): Promise<void> {
+		if (this.isStopped()) return;
+
+		const opened = await this.options.connect();
+
+		if (opened.err) {
+			this.options.log.warn('reconnect - could not open a socket', {
+				message: opened.err.message,
+			});
+			this.schedule();
+
+			return;
+		}
+
+		if (this.isStopped()) {
+			opened.sock.destroy();
+
+			return;
+		}
+
+		const up = await this.options.onConnected(opened.sock);
+
+		if (up.err) {
+			this.options.log.warn('reconnect - could not come back up', { message: up.err.message });
+			this.schedule();
+
+			return;
+		}
+
+		this.delay = this.options.minDelay;
+	}
+}
diff --git a/src/send-sms.ts b/src/send-sms.ts
new file mode 100644
index 0000000..bb1a6d9
--- /dev/null
+++ b/src/send-sms.ts
@@ -0,0 +1,106 @@
+import type { EncodingName } from './defs/encodings.ts';
+import type { LogInt } from '@larvit/log';
+import type { ParamValue } from './defs/types.ts';
+import type { PduObject, PduObjectInput } from './pdu.ts';
+import type { Result } from './result.ts';
+import { consts } from './defs/constants.ts';
+import { detect } from './defs/encodings.ts';
+import { paramText } from './defs/types.ts';
+import { smppTime, splitMessage } from './message.ts';
+
+export type SendSmsOptions = {
+	dlr?: boolean;
+	destinationAddrNpi?: number;
+	destinationAddrTon?: number;
+	encoding?: EncodingName;
+	flash?: boolean;
+	from: string;
+	message: string;
+	scheduleDeliveryTime?: Date | number | string;
+	sourceAddrNpi?: number;
+	sourceAddrTon?: number;
+	to: string;
+	validityPeriod?: Date | number | string;
+};
+
+export type SendSmsResult = Result<{ pduObjs: PduObject[]; smsIds: string[] }>;
+
+/** What sending needs from the session: a concat reference and a way onto the wire. */
+export type SendSmsDeps = {
+	log: LogInt;
+	reference: number;
+	send: (input: PduObjectInput) => Promise<Result<{ pduObj: PduObject }>>;
+};
+
+type SegmentOptions = {
+	encoding: EncodingName;
+	multipart: boolean;
+};
+
+/** Alphanumeric senders must be TON 5; 0.4.0 sent everything as TON 1 (international). */
+function addressTon(address: string): number {
+	return /^\+?\d+$/.test(address) ? consts.TON.INTERNATIONAL : consts.TON.ALPHANUMERIC;
+}
+
+function dataCodingFor(encoding: EncodingName, flash: boolean): number {
+	if (!flash) return consts.ENCODING[encoding];
+
+	// Message class present (0x10) plus the alphabet bits, so flash survives UCS2.
+	return encoding === 'UCS2' ? 0x18 : 0x10;
+}
+
+export function submitSmParams(
+	sms: SendSmsOptions,
+	segment: Buffer,
+	options: SegmentOptions,
+): Record<string, ParamValue> {
+	const params: Record<string, ParamValue> = {
+		data_coding: dataCodingFor(options.encoding, sms.flash === true),
+		destination_addr: sms.to,
+		dest_addr_npi: sms.destinationAddrNpi ?? 0,
+		dest_addr_ton: sms.destinationAddrTon ?? addressTon(sms.to),
+		short_message: segment,
+		sm_length: segment.length,
+		source_addr: sms.from,
+		source_addr_npi: sms.sourceAddrNpi ?? 0,
+		source_addr_ton: sms.sourceAddrTon ?? addressTon(sms.from),
+	};
+
+	if (options.multipart) params.esm_class = consts.ESM_CLASS.UDH_INDICATOR;
+	if (sms.dlr === true) params.registered_delivery = consts.REGISTERED_DELIVERY.FINAL;
+	if (sms.scheduleDeliveryTime !== undefined) {
+		params.schedule_delivery_time = smppTime.encode(sms.scheduleDeliveryTime);
+	}
+	if (sms.validityPeriod !== undefined) {
+		params.validity_period = smppTime.encode(sms.validityPeriod);
+	}
+
+	return params;
+}
+
+/** Puts a message on the wire as one submit_sm per segment. */
+export async function submitSms(deps: SendSmsDeps, sms: SendSmsOptions): Promise<SendSmsResult> {
+	const encoding = sms.encoding ?? detect(sms.message);
+	const segments = splitMessage(sms.message, { encoding, reference: deps.reference });
+	const multipart = segments.length > 1;
+	const pduObjs: PduObject[] = [];
+	const smsIds: string[] = [];
+
+	deps.log.debug('sendSms() - sending', { encoding, segments: segments.length, to: sms.to });
+
+	// Segments go out together rather than one-after-a-response: a receiver that waits for every
+	// segment before answering — this library's own server does — would otherwise deadlock.
+	const sent = await Promise.all(segments.map(segment => deps.send({
+		cmdName: 'submit_sm',
+		params: submitSmParams(sms, segment, { encoding, multipart }),
+	})));
+
+	for (const one of sent) {
+		if (one.err) return { err: one.err };
+
+		pduObjs.push(one.pduObj);
+		smsIds.push(paramText(one.pduObj.params.message_id));
+	}
+
+	return { pduObjs, smsIds };
+}
diff --git a/src/send-window.ts b/src/send-window.ts
new file mode 100644
index 0000000..6f2dea4
--- /dev/null
+++ b/src/send-window.ts
@@ -0,0 +1,32 @@
+/** Caps how many requests are on the wire at once; anything past the limit waits its turn. */
+export class SendWindow {
+	private readonly limit: number;
+	private readonly waiting: (() => void)[] = [];
+	private inFlight = 0;
+
+	constructor(limit: number) {
+		this.limit = limit;
+	}
+
+	acquire(): Promise<void> {
+		if (this.inFlight < this.limit) {
+			this.inFlight++;
+
+			return Promise.resolve();
+		}
+
+		return new Promise<void>(resolve => this.waiting.push(resolve));
+	}
+
+	release(): void {
+		const next = this.waiting.shift();
+
+		if (next) {
+			next();
+
+			return;
+		}
+
+		this.inFlight--;
+	}
+}
diff --git a/src/server.ts b/src/server.ts
index 3ad369f..7b5cf7e 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -4,12 +4,12 @@ import type { Result } from './result.ts';
 import type { Server as NetServer, Socket } from 'node:net';
 import type { Server as TlsServer, TlsOptions } from 'node:tls';
 import { EventEmitter } from 'node:events';
-import { Session } from './session.ts';
+import { Session, bindCommands } from './session.ts';
 import { createServer as createNetServer } from 'node:net';
 import { createServer as createTlsServer } from 'node:tls';
+import { defaultInterfaceVersion } from './defs/constants.ts';
 import { paramText } from './defs/types.ts';
 import { silentLog } from './log.ts';
-import { tlvs } from './defs/tlvs.ts';
 
 export type AuthenticateResult = { userData?: unknown } | boolean;
 
@@ -24,6 +24,7 @@ export type ServerOptions = {
 	authenticate?: (input: AuthenticateInput) => Promise<AuthenticateResult> | AuthenticateResult;
 	host?: string;
 	idleTimeout?: number;
+	interfaceVersion?: number;
 	log?: LogInt;
 	maxOutstanding?: number;
 	maxReassembly?: number;
@@ -31,6 +32,7 @@ export type ServerOptions = {
 	reassemblyTimeout?: number;
 	responseTimeout?: number;
 	signal?: AbortSignal;
+	systemId?: string;
 	tls?: TlsOptions | boolean;
 };
 
@@ -41,12 +43,11 @@ export type ServerEvents = {
 
 const defaults = {
 	idleTimeout: 40_000,
+	interfaceVersion: defaultInterfaceVersion,
 	port: 2775,
+	systemId: '',
 };
 
-const bindCommands = ['bind_receiver', 'bind_transceiver', 'bind_transmitter'];
-const scInterfaceVersion = 0x34;
-
 /** A listening SMPP server. Sessions arrive as `session` events; `close()` stops listening. */
 export class SmppServer extends EventEmitter<ServerEvents> {
 	readonly sessions = new Set<Session>();
@@ -102,25 +103,46 @@ async function authenticate(
 	return true;
 }
 
-/**
- * A peer that declared below 0x34 must not be sent optional parameters at all; above it, an ESME
- * reads a missing sc_interface_version as this SMSC having none.
- */
-function bindRespTlvs(pduObj: PduObject): Record<string, TlvInput> | undefined {
-	const declared = pduObj.params.interface_version;
-	const definition = tlvs.sc_interface_version;
-
-	if (!definition || typeof declared !== 'number' || declared < scInterfaceVersion) return undefined;
+/** An ESME reads a missing sc_interface_version as this SMSC having none. */
+function bindRespTlvs(session: Session, options: ServerOptions): Record<string, TlvInput> | undefined {
+	if (!session.acceptsOptionalParams()) return undefined;
 
 	return {
-		sc_interface_version: {
-			tagId: definition.id,
-			tagName: definition.tag,
-			tagValue: scInterfaceVersion,
-		},
+		sc_interface_version: { tagValue: options.interfaceVersion ?? defaults.interfaceVersion },
 	};
 }
 
+async function acceptBind(
+	session: Session,
+	pduObj: PduObject,
+	options: ServerOptions,
+	identity: Record<string, string>,
+): Promise<void> {
+	const declared = pduObj.params.interface_version;
+
+	session.loggedIn = true;
+	session.peerInterfaceVersion = typeof declared === 'number' ? declared : undefined;
+
+	await session.sendReturn(pduObj, 'ESME_ROK', identity, bindRespTlvs(session, options));
+}
+
+async function onBind(session: Session, pduObj: PduObject, options: ServerOptions): Promise<void> {
+	const log = options.log ?? silentLog;
+	// Explicit, or pduReturn's echo answers the ESME with its own system_id instead of ours.
+	const identity = { system_id: options.systemId ?? defaults.systemId };
+	const systemId = paramText(pduObj.params.system_id);
+
+	if (!await authenticate(session, pduObj, options)) {
+		log.info('server - bind refused', { systemId });
+		await session.sendReturn(pduObj, 'ESME_RBINDFAIL', identity);
+
+		return;
+	}
+
+	await acceptBind(session, pduObj, options, identity);
+	log.verbose('server - bound', { systemId });
+}
+
 /**
  * Handles everything a peer may send before it is bound. Returns true when it has answered, so the
  * session leaves the PDU alone.
@@ -130,27 +152,18 @@ async function onRequest(
 	pduObj: PduObject,
 	options: ServerOptions,
 ): Promise<boolean> {
-	const log = options.log ?? silentLog;
-
 	if (session.loggedIn || pduObj.cmdName === 'unbind') return false;
 
 	if (!bindCommands.includes(pduObj.cmdName)) {
+		const log = options.log ?? silentLog;
+
 		log.debug('server - command before bind', { cmdName: pduObj.cmdName });
 		await session.sendReturn(pduObj, 'ESME_RINVBNDSTS');
 
 		return true;
 	}
 
-	if (!await authenticate(session, pduObj, options)) {
-		log.info('server - bind refused', { systemId: paramText(pduObj.params.system_id) });
-		await session.sendReturn(pduObj, 'ESME_RBINDFAIL');
-
-		return true;
-	}
-
-	session.loggedIn = true;
-	await session.sendReturn(pduObj, 'ESME_ROK', {}, bindRespTlvs(pduObj));
-	log.verbose('server - bound', { systemId: paramText(pduObj.params.system_id) });
+	await onBind(session, pduObj, options);
 
 	return true;
 }
@@ -166,6 +179,7 @@ function onConnection(sock: Socket, options: ServerOptions, server: SmppServer):
 		reassemblyTimeout: options.reassemblyTimeout,
 		responseTimeout: options.responseTimeout,
 		sock,
+		systemId: options.systemId ?? defaults.systemId,
 	});
 
 	server.sessions.add(session);
@@ -190,28 +204,58 @@ function createSecureListener(tlsOptions: TlsOptions, log: LogInt): TlsServer {
 	return listener;
 }
 
+function createListener(
+	options: ServerOptions,
+	log: LogInt,
+	port: number,
+): Result<{ listener: NetServer | TlsServer; useTls: boolean }> {
+	const useTls = options.tls !== undefined && options.tls !== false;
+	const tlsOptions = typeof options.tls === 'object' ? options.tls : undefined;
+
+	if (useTls && !tlsOptions) {
+		log.warn('server - tls without a certificate', { port });
+
+		return { err: new Error('Listening over TLS needs tls: { cert, key }') };
+	}
+
+	return {
+		listener: tlsOptions ? createSecureListener(tlsOptions, log) : createNetServer(),
+		useTls,
+	};
+}
+
+function onListening(listener: NetServer, smpp: SmppServer, options: ServerOptions): void {
+	const log = options.log ?? silentLog;
+
+	// Past startup, a listener error is a runtime event, not a failed start.
+	listener.on('error', (err: Error) => {
+		log.warn('server - error', { message: err.message });
+		smpp.emit('serverError', err);
+	});
+
+	log.info('server - listening', { host: options.host ?? '*', port: smpp.port });
+
+	if (options.signal) {
+		options.signal.addEventListener('abort', () => { void smpp.close(); }, { once: true });
+	}
+}
+
 /** Starts listening for SMPP connections. Resolves once the socket is bound. */
 export function server(options: ServerOptions = {}): Promise<Result<{ server: SmppServer }>> {
+	const log = options.log ?? silentLog;
+	const port = options.port ?? defaults.port;
+	const created = createListener(options, log, port);
+
+	if (created.err) return Promise.resolve({ err: created.err });
+
+	const listener = created.listener;
+	const smpp = new SmppServer(listener);
+
+	listener.on(created.useTls ? 'secureConnection' : 'connection', (sock: Socket) => {
+		onConnection(sock, options, smpp);
+	});
+
 	return new Promise(resolve => {
-		const log = options.log ?? silentLog;
-		const port = options.port ?? defaults.port;
-		const useTls = options.tls !== undefined && options.tls !== false;
-		const tlsOptions = typeof options.tls === 'object' ? options.tls : undefined;
-
-		if (useTls && !tlsOptions) {
-			log.warn('server - tls without a certificate', { port });
-			resolve({ err: new Error('Listening over TLS needs tls: { cert, key }') });
-
-			return;
-		}
-
-		const listener = tlsOptions ? createSecureListener(tlsOptions, log) : createNetServer();
-		const smpp = new SmppServer(listener);
-
-		listener.on(useTls ? 'secureConnection' : 'connection', (sock: Socket) => {
-			onConnection(sock, options, smpp);
-		});
-
 		const onStartupError = (err: Error): void => {
 			listener.removeListener('error', onStartupError);
 			log.warn('server - could not listen', { message: err.message, port });
@@ -222,19 +266,7 @@ export function server(options: ServerOptions = {}): Promise<Result<{ server: Sm
 
 		listener.listen(port, options.host, () => {
 			listener.removeListener('error', onStartupError);
-
-			// Past startup, a listener error is a runtime event, not a failed start.
-			listener.on('error', (err: Error) => {
-				log.warn('server - error', { message: err.message });
-				smpp.emit('serverError', err);
-			});
-
-			log.info('server - listening', { host: options.host ?? '*', port: smpp.port });
-
-			if (options.signal) {
-				options.signal.addEventListener('abort', () => { void smpp.close(); }, { once: true });
-			}
-
+			onListening(listener, smpp, options);
 			resolve({ server: smpp });
 		});
 	});
diff --git a/src/session.ts b/src/session.ts
index d596ef7..83837e1 100644
--- a/src/session.ts
+++ b/src/session.ts
@@ -1,25 +1,31 @@
 import type { Dlr } from './dlr.ts';
-import type { EncodingName } from './defs/encodings.ts';
 import type { ErrorName } from './defs/errors.ts';
 import type { LogInt } from '@larvit/log';
+import type { MessageDlr } from './dlr-merger.ts';
 import type { ParamValue } from './defs/types.ts';
 import type { PduObject, PduObjectInput, TlvInput } from './pdu.ts';
 import type { Result, VoidResult } from './result.ts';
+import type { SendSmsOptions, SendSmsResult } from './send-sms.ts';
 import type { Sms } from './sms.ts';
 import type { Socket } from 'node:net';
+import { DlrMerger } from './dlr-merger.ts';
 import { EventEmitter } from 'node:events';
+import { LinkTimers } from './link-timers.ts';
 import { PduFramer } from './pdu-framer.ts';
+import { PendingRequests } from './pending-requests.ts';
+import { ReconnectLoop } from './reconnect-loop.ts';
+import { Reassembler, decodeSegments } from './reassembly.ts';
+import { SendWindow } from './send-window.ts';
 import { concatInfo } from './udh.ts';
-import { consts } from './defs/constants.ts';
+import { consts, optionalParamsMinVersion } from './defs/constants.ts';
 import { createSms } from './sms.ts';
-import { decodeMessage, smppTime, splitMessage } from './message.ts';
-import { detect } from './defs/encodings.ts';
 import { dlrFromPdu } from './dlr.ts';
-import { isResp, maxSeqNr, objToPdu, pduReturn, pduToObj } from './pdu.ts';
+import { isResp, objToPdu, pduReturn, pduToObj } from './pdu.ts';
 import { paramText } from './defs/types.ts';
 import { silentLog } from './log.ts';
+import { submitSms } from './send-sms.ts';
 
-export type MessageDlr = Dlr & { segments: Dlr[] };
+export type { MessageDlr, SendSmsOptions };
 
 export type SessionEvents = {
 	close: [];
@@ -46,21 +52,6 @@ export type ReconnectOptions = {
 	onConnected: (session: Session) => Promise<VoidResult>;
 };
 
-export type SendSmsOptions = {
-	dlr?: boolean;
-	destinationAddrNpi?: number;
-	destinationAddrTon?: number;
-	encoding?: EncodingName;
-	flash?: boolean;
-	from: string;
-	message: string;
-	scheduleDeliveryTime?: Date | number | string;
-	sourceAddrNpi?: number;
-	sourceAddrTon?: number;
-	to: string;
-	validityPeriod?: Date | number | string;
-};
-
 export type SessionOptions = {
 	enquireLinkInterval?: number | undefined;
 	idleTimeout?: number | undefined;
@@ -77,16 +68,8 @@ export type SessionOptions = {
 	reconnect?: ReconnectOptions | undefined;
 	responseTimeout?: number | undefined;
 	sock: Socket;
-};
-
-type Pending = {
-	settle: (result: Result<{ pduObj: PduObject }>) => void;
-};
-
-type Reassembly = {
-	parts: Map<number, PduObject>;
-	timer: NodeJS.Timeout;
-	total: number;
+	/** This end's own identity, answered to the peer in place of the one it sent. */
+	systemId?: string | undefined;
 };
 
 const defaults = {
@@ -96,19 +79,14 @@ const defaults = {
 	minDelay: 1000,
 	reassemblyTimeout: 300_000,
 	responseTimeout: 30_000,
+	systemId: '',
 };
 
-/** Alphanumeric senders must be TON 5; 0.4.0 sent everything as TON 1 (international). */
-function addressTon(address: string): number {
-	return /^\+?\d+$/.test(address) ? consts.TON.INTERNATIONAL : consts.TON.ALPHANUMERIC;
-}
-
-function dataCodingFor(encoding: EncodingName, flash: boolean): number {
-	if (!flash) return consts.ENCODING[encoding];
-
-	// Message class present (0x10) plus the alphabet bits, so flash survives UCS2.
-	return encoding === 'UCS2' ? 0x18 : 0x10;
-}
+export const bindCommands: readonly string[] = [
+	'bind_receiver',
+	'bind_transceiver',
+	'bind_transmitter',
+];
 
 export class Session extends EventEmitter<SessionEvents> {
 	/** Replaced on reconnect, so hold the session rather than this. */
@@ -116,50 +94,52 @@ export class Session extends EventEmitter<SessionEvents> {
 	readonly log: LogInt;
 
 	loggedIn = false;
+	/** The interface_version the peer declared when binding; undefined until a bind is accepted. */
+	peerInterfaceVersion: number | undefined = undefined;
 	userData: unknown = undefined;
 
-	private framer = new PduFramer();
+	private readonly dlrMerger = new DlrMerger();
 	private readonly options: SessionOptions;
-	private readonly pending = new Map<number, Pending>();
-	private readonly reassembly = new Map<string, Reassembly>();
-	private readonly segmentDlrs = new Map<string, { expected: number; parts: Map<number, Dlr> }>();
-	private readonly waiting: (() => void)[] = [];
+	private readonly pending: PendingRequests;
+	private readonly reassembler: Reassembler;
+	private readonly reconnectLoop: ReconnectLoop | undefined;
+	private readonly timers: LinkTimers;
+	private readonly window: SendWindow;
 
 	private closed = false;
 	private concatReference = 0;
-	private enquireLinkTimer: NodeJS.Timeout | undefined;
-	private idleTimer: NodeJS.Timeout | undefined;
-	private inFlight = 0;
-	private ourSeqNr = 1;
-	private reconnectDelay: number;
-	private reconnectTimer: NodeJS.Timeout | undefined;
-	private stopped = false;
+	private framer = new PduFramer();
 
 	constructor(options: SessionOptions) {
 		super();
 
-		this.options = options;
 		this.log = options.log ?? silentLog;
+		this.options = options;
+		this.pending = new PendingRequests(this.log);
+		this.reassembler = new Reassembler({
+			log: this.log,
+			max: options.maxReassembly ?? defaults.maxReassembly,
+			timeout: options.reassemblyTimeout ?? defaults.reassemblyTimeout,
+		});
+		this.reconnectLoop = this.loopFor(options.reconnect);
 		this.sock = options.sock;
-		this.reconnectDelay = options.reconnect?.minDelay ?? defaults.minDelay;
+		this.timers = new LinkTimers({
+			enquireLinkInterval: options.enquireLinkInterval,
+			idleTimeout: options.idleTimeout,
+			log: this.log,
+			onEnquireLink: () => { void this.send({ cmdName: 'enquire_link' }); },
+			onIdle: () => { this.close(); },
+		});
+		this.window = new SendWindow(options.maxOutstanding ?? defaults.maxOutstanding);
 
 		this.attach(options.sock);
 		this.resetTimers();
 	}
 
-	/** Wires a freshly opened socket into this session, replacing any previous one. */
-	private attach(sock: Socket): void {
-		this.sock = sock;
-		this.framer = new PduFramer();
-		this.closed = false;
-
-		sock.on('data', chunk => { this.onData(chunk); });
-		sock.on('close', () => { this.onClose(); });
-		sock.on('error', err => {
-			this.log.warn('session - socket error', { message: err.message });
-			this.emit('sessionError', err);
-			this.onClose();
-		});
+	/** SMPP 3.4 forbids sending optional parameters to a peer that declared an older version. */
+	acceptsOptionalParams(): boolean {
+		return this.peerInterfaceVersion === undefined
+			|| this.peerInterfaceVersion >= optionalParamsMinVersion;
 	}
 
 	/** Sends a request and resolves with the peer's response. */
@@ -173,26 +153,12 @@ export class Session extends EventEmitter<SessionEvents> {
 
 		if (this.closed) return { err: new Error('Session is closed') };
 
-		await this.acquire();
+		await this.window.acquire();
 
 		try {
-			const seqNr = this.nextSeqNr();
-			const built = objToPdu({ ...input, seqNr });
-
-			if (built.err) return { err: built.err };
-
-			const response = this.awaitResponse(seqNr, options.signal);
-			const written = this.write(built.buffer);
-
-			if (written.err) {
-				this.settle(seqNr, { err: written.err });
-
-				return { err: written.err };
-			}
-
-			return await response;
+			return await this.request(input, options);
 		} finally {
-			this.release();
+			this.window.release();
 		}
 	}
 
@@ -210,57 +176,16 @@ export class Session extends EventEmitter<SessionEvents> {
 		return Promise.resolve(this.write(built.buffer));
 	}
 
-	async sendSms(
-		sms: SendSmsOptions,
-		options: SendOptions = {},
-	): Promise<Result<{ pduObjs: PduObject[]; smsIds: string[] }>> {
-		const encoding = sms.encoding ?? detect(sms.message);
-		const segments = splitMessage(sms.message, {
-			encoding,
+	async sendSms(sms: SendSmsOptions, options: SendOptions = {}): Promise<SendSmsResult> {
+		const sent = await submitSms({
+			log: this.log,
 			reference: this.nextConcatReference(),
-		});
-		const pduObjs: PduObject[] = [];
-		const smsIds: string[] = [];
+			send: input => this.send(input, options),
+		}, sms);
 
-		this.log.debug('sendSms() - sending', { encoding, segments: segments.length, to: sms.to });
+		if (!sent.err) this.dlrMerger.expect(sent.smsIds);
 
-		// Segments go out together rather than one-after-a-response: a receiver that waits for every
-		// segment before answering — this library's own server does — would otherwise deadlock.
-		const sent = await Promise.all(segments.map(segment => {
-			const params: Record<string, ParamValue> = {
-				data_coding: dataCodingFor(encoding, sms.flash === true),
-				destination_addr: sms.to,
-				dest_addr_npi: sms.destinationAddrNpi ?? 0,
-				dest_addr_ton: sms.destinationAddrTon ?? addressTon(sms.to),
-				short_message: segment,
-				sm_length: segment.length,
-				source_addr: sms.from,
-				source_addr_npi: sms.sourceAddrNpi ?? 0,
-				source_addr_ton: sms.sourceAddrTon ?? addressTon(sms.from),
-			};
-
-			if (segments.length > 1) params.esm_class = consts.ESM_CLASS.UDH_INDICATOR;
-			if (sms.dlr === true) params.registered_delivery = consts.REGISTERED_DELIVERY.FINAL;
-			if (sms.validityPeriod !== undefined) {
-				params.validity_period = smppTime.encode(sms.validityPeriod);
-			}
-			if (sms.scheduleDeliveryTime !== undefined) {
-				params.schedule_delivery_time = smppTime.encode(sms.scheduleDeliveryTime);
-			}
-
-			return this.send({ cmdName: 'submit_sm', params }, options);
-		}));
-
-		for (const one of sent) {
-			if (one.err) return { err: one.err };
-
-			pduObjs.push(one.pduObj);
-			smsIds.push(paramText(one.pduObj.params.message_id));
-		}
-
-		this.expectSegmentDlrs(smsIds);
-
-		return { pduObjs, smsIds };
+		return sent;
 	}
 
 	/** Unbinds politely, then closes. */
@@ -274,71 +199,99 @@ export class Session extends EventEmitter<SessionEvents> {
 
 	/** Closes for good. A session closed this way never reconnects. */
 	close(): void {
-		this.stopped = true;
-
-		if (this.reconnectTimer) clearTimeout(this.reconnectTimer);
-
-		this.reconnectTimer = undefined;
+		this.reconnectLoop?.stop();
 		this.teardown();
 	}
 
+	private loopFor(reconnect: ReconnectOptions | undefined): ReconnectLoop | undefined {
+		if (!reconnect) return undefined;
+
+		return new ReconnectLoop({
+			connect: reconnect.connect,
+			log: this.log,
+			maxDelay: reconnect.maxDelay ?? defaults.maxDelay,
+			minDelay: reconnect.minDelay ?? defaults.minDelay,
+			onConnected: sock => this.comeBackUp(sock, reconnect.onConnected),
+		});
+	}
+
+	private async comeBackUp(
+		sock: Socket,
+		bind: (session: Session) => Promise<VoidResult>,
+	): Promise<VoidResult> {
+		this.attach(sock);
+
+		const bound = await bind(this);
+
+		if (bound.err) {
+			this.teardown();
+
+			return { err: bound.err };
+		}
+
+		this.resetTimers();
+		this.log.info('session - reconnected');
+		this.emit('reconnected');
+
+		return {};
+	}
+
+	/** Wires a freshly opened socket into this session, replacing any previous one. */
+	private attach(sock: Socket): void {
+		this.sock = sock;
+		this.framer = new PduFramer();
+		this.closed = false;
+
+		sock.on('data', chunk => { this.onData(chunk); });
+		sock.on('close', () => { this.onClose(); });
+		sock.on('error', err => {
+			this.log.warn('session - socket error', { message: err.message });
+			this.emit('sessionError', err);
+			this.onClose();
+		});
+	}
+
+	private async request(
+		input: PduObjectInput,
+		options: SendOptions,
+	): Promise<Result<{ pduObj: PduObject }>> {
+		const seqNr = this.pending.nextSeqNr();
+		const built = objToPdu({ ...input, seqNr });
+
+		if (built.err) return { err: built.err };
+
+		const response = this.pending.wait(seqNr, {
+			signal: options.signal,
+			timeout: this.options.responseTimeout ?? defaults.responseTimeout,
+		});
+		const written = this.write(built.buffer);
+
+		if (written.err) {
+			this.pending.settle(seqNr, { err: written.err });
+
+			return { err: written.err };
+		}
+
+		return response;
+	}
+
 	private teardown(): void {
 		if (this.closed) return;
 
 		this.closed = true;
-		this.clearTimers();
-
-		for (const [seqNr] of this.pending) {
-			this.settle(seqNr, { err: new Error('Session closed before a response arrived') });
-		}
-
-		for (const group of this.reassembly.values()) {
-			clearTimeout(group.timer);
-		}
-
-		this.reassembly.clear();
+		this.timers.clear();
+		this.pending.settleAll(new Error('Session closed before a response arrived'));
+		this.reassembler.clear();
 		this.sock.destroy();
 		this.emit('close');
 	}
 
-	private nextSeqNr(): number {
-		const seqNr = this.ourSeqNr;
-
-		this.ourSeqNr = this.ourSeqNr >= maxSeqNr ? 1 : this.ourSeqNr + 1;
-
-		return seqNr;
-	}
-
 	private nextConcatReference(): number {
 		this.concatReference = this.concatReference >= 255 ? 1 : this.concatReference + 1;
 
 		return this.concatReference;
 	}
 
-	private async acquire(): Promise<void> {
-		const limit = this.options.maxOutstanding ?? defaults.maxOutstanding;
-
-		if (this.inFlight < limit) {
-			this.inFlight++;
-
-			return;
-		}
-
-		return new Promise<void>(resolve => this.waiting.push(resolve));
-	}
-
-	private release(): void {
-		const next = this.waiting.shift();
-
-		if (next) {
-			next();
-
-			return;
-		}
-
-		this.inFlight--;
-	}
-
 	private write(pdu: Buffer): VoidResult {
 		if (this.sock.destroyed) {
 			return { err: new Error('Socket is closed') };
@@ -349,49 +302,6 @@ export class Session extends EventEmitter<SessionEvents> {
 		return {};
 	}
 
-	private awaitResponse(
-		seqNr: number,
-		signal: AbortSignal | undefined,
-	): Promise<Result<{ pduObj: PduObject }>> {
-		return new Promise(resolve => {
-			const timeout = this.options.responseTimeout ?? defaults.responseTimeout;
-			let timer: NodeJS.Timeout | undefined;
-
-			const onAbort = (): void => {
-				this.settle(seqNr, { err: new Error('Aborted before a response arrived') });
-			};
-
-			const settle = (result: Result<{ pduObj: PduObject }>): void => {
-				if (timer) clearTimeout(timer);
-				signal?.removeEventListener('abort', onAbort);
-				this.pending.delete(seqNr);
-				resolve(result);
-			};
-
-			this.pending.set(seqNr, { settle });
-
-			if (signal?.aborted === true) {
-				settle({ err: new Error('Aborted before a response arrived') });
-
-				return;
-			}
-
-			signal?.addEventListener('abort', onAbort, { once: true });
-
-			if (timeout > 0) {
-				timer = setTimeout(() => {
-					this.log.warn('session - no response before the timeout', { seqNr, timeout });
-					this.settle(seqNr, { err: new Error(`No response to seqNr ${String(seqNr)}`) });
-				}, timeout);
-				timer.unref();
-			}
-		});
-	}
-
-	private settle(seqNr: number, result: Result<{ pduObj: PduObject }>): void {
-		this.pending.get(seqNr)?.settle(result);
-	}
-
 	private onData(chunk: Buffer): void {
 		this.emit('data', chunk);
 		this.resetTimers();
@@ -408,36 +318,37 @@ export class Session extends EventEmitter<SessionEvents> {
 		}
 
 		for (const pdu of framed.pdus) {
-			this.emit('incomingPdu', pdu);
-
-			const parsed = pduToObj(pdu);
-
-			if (parsed.err) {
-				this.log.warn('session - could not parse an incoming PDU, closing', {
-					message: parsed.err.message,
-				});
-				this.emit('sessionError', parsed.err);
-				this.close();
-
-				return;
-			}
-
-			this.dispatch(parsed.pduObj);
+			if (!this.receive(pdu)) return;
 		}
 	}
 
+	/** False means the PDU could not be read and the session has been closed. */
+	private receive(pdu: Buffer): boolean {
+		this.emit('incomingPdu', pdu);
+
+		const parsed = pduToObj(pdu);
+
+		if (parsed.err) {
+			this.log.warn('session - could not parse an incoming PDU, closing', {
+				message: parsed.err.message,
+			});
+			this.emit('sessionError', parsed.err);
+			this.close();
+
+			return false;
+		}
+
+		this.dispatch(parsed.pduObj);
+
+		return true;
+	}
+
 	private dispatch(pduObj: PduObject): void {
 		if (isResp(pduObj)) {
-			const pending = this.pending.get(pduObj.seqNr);
-
-			if (!pending) {
+			if (!this.pending.deliver(pduObj)) {
 				this.log.debug('session - response with no matching request', { seqNr: pduObj.seqNr });
-
-				return;
 			}
 
-			pending.settle({ pduObj });
-
 			return;
 		}
 
@@ -450,27 +361,32 @@ export class Session extends EventEmitter<SessionEvents> {
 
 		if (onRequest && await onRequest(this, pduObj)) return;
 
-		if (pduObj.cmdName === 'enquire_link') {
-			await this.sendReturn(pduObj);
-
-			return;
+		switch (pduObj.cmdName) {
+			case 'deliver_sm':
+				await this.onDeliverSm(pduObj);
+				break;
+			case 'enquire_link':
+				await this.sendReturn(pduObj);
+				break;
+			case 'submit_sm':
+				this.onSubmitSm(pduObj);
+				break;
+			case 'unbind':
+				await this.sendReturn(pduObj);
+				this.close();
+				break;
+			default:
+				await this.unhandled(pduObj);
 		}
+	}
 
-		if (pduObj.cmdName === 'unbind') {
-			await this.sendReturn(pduObj);
-			this.close();
-
-			return;
-		}
-
-		if (pduObj.cmdName === 'submit_sm') {
-			this.onSubmitSm(pduObj);
-
-			return;
-		}
-
-		if (pduObj.cmdName === 'deliver_sm') {
-			await this.onDeliverSm(pduObj);
+	private async unhandled(pduObj: PduObject): Promise<void> {
+		if (bindCommands.includes(pduObj.cmdName)) {
+			this.log.info('session - bind on an already bound session', { cmdName: pduObj.cmdName });
+			// Explicit, or pduReturn's echo answers the peer with its own system_id instead of ours.
+			await this.sendReturn(pduObj, 'ESME_RALYBND', {
+				system_id: this.options.systemId ?? defaults.systemId,
+			});
 
 			return;
 		}
@@ -484,14 +400,7 @@ export class Session extends EventEmitter<SessionEvents> {
 		const esmClass = pduObj.params.esm_class;
 		const hasUdh = typeof esmClass === 'number'
 			&& (esmClass & consts.ESM_CLASS.UDH_INDICATOR) === consts.ESM_CLASS.UDH_INDICATOR;
-
-		if (!hasUdh || !Buffer.isBuffer(message)) {
-			this.emitSms([pduObj]);
-
-			return;
-		}
-
-		const concat = concatInfo(message);
+		const concat = hasUdh && Buffer.isBuffer(message) ? concatInfo(message) : undefined;
 
 		if (!concat) {
 			this.emitSms([pduObj]);
@@ -499,66 +408,9 @@ export class Session extends EventEmitter<SessionEvents> {
 			return;
 		}
 
-		this.collectSegment(pduObj, concat);
-	}
+		const whole = this.reassembler.collect(pduObj, concat);
 
-	private collectSegment(
-		pduObj: PduObject,
-		concat: { part: number; reference: number; total: number },
-	): void {
-		const key = [
-			paramText(pduObj.params.source_addr),
-			paramText(pduObj.params.destination_addr),
-			String(concat.reference),
-		].join('_');
-
-		let group = this.reassembly.get(key);
-
-		if (!group) {
-			const limit = this.options.maxReassembly ?? defaults.maxReassembly;
-
-			if (this.reassembly.size >= limit) {
-				const oldest = this.reassembly.keys().next();
-
-				if (!oldest.done) {
-					this.log.warn('session - reassembly buffer full, dropping the oldest message', {
-						limit,
-					});
-					this.dropReassembly(oldest.value);
-				}
-			}
-
-			const timer = setTimeout(() => {
-				this.log.info('session - incomplete message expired', { key, total: concat.total });
-				this.dropReassembly(key);
-			}, this.options.reassemblyTimeout ?? defaults.reassemblyTimeout);
-
-			timer.unref();
-			group = { parts: new Map(), timer, total: concat.total };
-			this.reassembly.set(key, group);
-		}
-
-		group.parts.set(concat.part, pduObj);
-
-		if (group.parts.size < group.total) return;
-
-		clearTimeout(group.timer);
-		this.reassembly.delete(key);
-
-		const ordered = [...group.parts.entries()]
-			.sort(([a], [b]) => a - b)
-			.map(([, part]) => part);
-
-		this.emitSms(ordered);
-	}
-
-	private dropReassembly(key: string): void {
-		const group = this.reassembly.get(key);
-
-		if (!group) return;
-
-		clearTimeout(group.timer);
-		this.reassembly.delete(key);
+		if (whole) this.emitSms(whole);
 	}
 
 	private emitSms(pduObjs: PduObject[]): void {
@@ -566,25 +418,9 @@ export class Session extends EventEmitter<SessionEvents> {
 
 		if (!first) return;
 
-		let message = '';
-
-		for (const pduObj of pduObjs) {
-			const part = pduObj.params.short_message;
-			const dataCoding = pduObj.params.data_coding;
-			const esmClass = pduObj.params.esm_class;
-
-			message += Buffer.isBuffer(part)
-				? decodeMessage(
-					part,
-					typeof dataCoding === 'number' ? dataCoding : 0,
-					typeof esmClass === 'number' ? esmClass : 0,
-				).message
-				: paramText(part);
-		}
-
 		this.emit('sms', createSms({
 			from: paramText(first.params.source_addr),
-			message,
+			message: decodeSegments(pduObjs),
 			pduObjs,
 			session: this,
 			to: paramText(first.params.destination_addr),
@@ -602,158 +438,22 @@ export class Session extends EventEmitter<SessionEvents> {
 		}
 
 		this.emit('dlr', dlr, pduObj);
-		this.collectSegmentDlr(dlr);
+
+		const merged = this.dlrMerger.collect(dlr);
+
+		if (merged) this.emit('messageDlr', merged);
+
 		await this.sendReturn(pduObj);
 	}
 
-	/**
-	 * Registers a multipart message for merged reporting, but only when the peer numbered its ids
-	 * `<base>-<n>` off one base — the convention this library's own server follows. An SMSC that
-	 * hands out unrelated ids per segment cannot be merged, so no messageDlr is emitted for it.
-	 */
-	private expectSegmentDlrs(smsIds: string[]): void {
-		if (smsIds.length < 2) return;
-
-		const bases = new Set<string>();
-
-		for (const smsId of smsIds) {
-			const match = /^(.*)-(\d+)$/.exec(smsId);
-
-			if (!match?.[1]) return;
-
-			bases.add(match[1]);
-		}
-
-		if (bases.size !== 1) return;
-
-		for (const base of bases) {
-			this.segmentDlrs.set(base, { expected: smsIds.length, parts: new Map() });
-		}
-	}
-
-	/** Collects per-segment receipts and reports once on the whole message. */
-	private collectSegmentDlr(dlr: Dlr): void {
-		const match = /^(.*)-(\d+)$/.exec(dlr.smsId);
-		const base = match?.[1];
-		const part = match?.[2];
-
-		if (base === undefined || part === undefined) return;
-
-		const group = this.segmentDlrs.get(base);
-
-		if (!group) return;
-
-		group.parts.set(Number(part), dlr);
-
-		if (group.parts.size < group.expected) return;
-
-		this.segmentDlrs.delete(base);
-
-		const ordered = [...group.parts.entries()].sort(([a], [b]) => a - b).map(([, one]) => one);
-		const worst = ordered.reduce((carry, one) => (one.statusId > carry.statusId ? one : carry));
-
-		this.emit('messageDlr', { ...worst, segments: ordered, smsId: base });
-	}
-
 	private resetTimers(): void {
 		if (this.closed) return;
 
-		this.clearTimers();
-
-		const { enquireLinkInterval, idleTimeout } = this.options;
-
-		if (enquireLinkInterval !== undefined && enquireLinkInterval > 0) {
-			this.enquireLinkTimer = setTimeout(() => {
-				void this.send({ cmdName: 'enquire_link' });
-			}, enquireLinkInterval);
-			this.enquireLinkTimer.unref();
-		}
-
-		if (idleTimeout !== undefined && idleTimeout > 0) {
-			this.idleTimer = setTimeout(() => {
-				this.log.info('session - closing an idle peer', { idleTimeout });
-				this.close();
-			}, idleTimeout);
-			this.idleTimer.unref();
-		}
-	}
-
-	private clearTimers(): void {
-		if (this.enquireLinkTimer) clearTimeout(this.enquireLinkTimer);
-		if (this.idleTimer) clearTimeout(this.idleTimer);
-
-		this.enquireLinkTimer = undefined;
-		this.idleTimer = undefined;
+		this.timers.reset();
 	}
 
 	private onClose(): void {
 		this.teardown();
-
-		if (!this.stopped && this.options.reconnect) this.scheduleReconnect();
-	}
-
-	private scheduleReconnect(): void {
-		if (this.reconnectTimer) return;
-
-		const reconnect = this.options.reconnect;
-
-		if (!reconnect) return;
-
-		const delay = this.reconnectDelay;
-
-		this.log.info('session - reconnecting after a drop', { delay });
-
-		this.reconnectTimer = setTimeout(() => {
-			this.reconnectTimer = undefined;
-			void this.reconnect();
-		}, delay);
-		this.reconnectTimer.unref();
-
-		this.reconnectDelay = Math.min(delay * 2, reconnect.maxDelay ?? defaults.maxDelay);
-	}
-
-	/** Read through a method: close() can land while a reconnect is awaiting. */
-	private isStopped(): boolean {
-		return this.stopped;
-	}
-
-	private async reconnect(): Promise<void> {
-		const reconnect = this.options.reconnect;
-
-		if (!reconnect || this.isStopped()) return;
-
-		const opened = await reconnect.connect();
-
-		if (opened.err) {
-			this.log.warn('session - reconnect failed to open a socket', {
-				message: opened.err.message,
-			});
-			this.scheduleReconnect();
-
-			return;
-		}
-
-		if (this.isStopped()) {
-			opened.sock.destroy();
-
-			return;
-		}
-
-		this.attach(opened.sock);
-
-		const bound = await reconnect.onConnected(this);
-
-		if (bound.err) {
-			this.log.warn('session - reconnect failed to bind', { message: bound.err.message });
-			this.teardown();
-			this.scheduleReconnect();
-
-			return;
-		}
-
-		this.reconnectDelay = reconnect.minDelay ?? defaults.minDelay;
-		this.resetTimers();
-		this.log.info('session - reconnected');
-		this.emit('reconnected');
+		this.reconnectLoop?.schedule();
 	}
 }
diff --git a/src/sms.ts b/src/sms.ts
index d477e57..e8b3ba3 100644
--- a/src/sms.ts
+++ b/src/sms.ts
@@ -1,6 +1,6 @@
 import type { ErrorName } from './defs/errors.ts';
 import type { MessageState } from './defs/constants.ts';
-import type { PduObject } from './pdu.ts';
+import type { PduObject, TlvInput } from './pdu.ts';
 import type { Result, VoidResult } from './result.ts';
 import type { Session } from './session.ts';
 import { consts } from './defs/constants.ts';
@@ -80,44 +80,47 @@ async function sendResp(sms: Sms, status: ErrorName = 'ESME_ROK'): Promise<VoidR
 	return results.find(result => result.err) ?? {};
 }
 
+/** The receipt as text, which is all of it a peer below SMPP 3.4 is allowed to be sent. */
+function receiptText(sms: Sms, smsId: string, status: MessageState): string {
+	const delivered = status === 'DELIVERED';
+
+	return [
+		`id:${smsId}`,
+		'sub:001',
+		`dlvrd:${delivered ? '001' : '000'}`,
+		`submit date:${smppDate(sms.submitTime)}`,
+		`done date:${smppDate(new Date())}`,
+		`stat:${receiptCodes[status]}`,
+		`err:${delivered ? '000' : '001'}`,
+		'text:',
+	].join(' ');
+}
+
+function receiptTlvs(smsId: string, status: MessageState): Record<string, TlvInput> {
+	return {
+		message_state: { tagValue: consts.MESSAGE_STATE[status] },
+		receipted_message_id: { tagValue: smsId },
+	};
+}
+
 async function sendDlr(
 	sms: Sms,
 	status: MessageState = 'DELIVERED',
 ): Promise<Result<{ pduObjs: PduObject[] }>> {
-	const statusId = consts.MESSAGE_STATE[status];
 	const total = sms.pduObjs.length;
 	const pduObjs: PduObject[] = [];
 
 	for (let index = 0; index < total; index++) {
 		const smsId = segmentId(sms.smsId, index, total);
-		const delivered = status === 'DELIVERED';
-		const message = [
-			`id:${smsId}`,
-			'sub:001',
-			`dlvrd:${delivered ? '001' : '000'}`,
-			`submit date:${smppDate(sms.submitTime)}`,
-			`done date:${smppDate(new Date())}`,
-			`stat:${receiptCodes[status]}`,
-			`err:${delivered ? '000' : '001'}`,
-			'text:',
-		].join(' ');
-
 		const sent = await sms.session.send({
 			cmdName: 'deliver_sm',
 			params: {
 				destination_addr: sms.from,
 				esm_class: consts.ESM_CLASS.MC_DELIVERY_RECEIPT,
-				short_message: message,
+				short_message: receiptText(sms, smsId, status),
 				source_addr: sms.to,
 			},
-			tlvs: {
-				message_state: { tagId: 0x0427, tagName: 'message_state', tagValue: statusId },
-				receipted_message_id: {
-					tagId: 0x001E,
-					tagName: 'receipted_message_id',
-					tagValue: smsId,
-				},
-			},
+			...(sms.session.acceptsOptionalParams() ? { tlvs: receiptTlvs(smsId, status) } : {}),
 		});
 
 		if (sent.err) return { err: sent.err };
diff --git a/test/dlr.test.ts b/test/dlr.test.ts
index b0fbc26..0bbadc6 100644
--- a/test/dlr.test.ts
+++ b/test/dlr.test.ts
@@ -61,8 +61,8 @@ describe('parseReceipt()', () => {
 describe('dlrFromPdu()', () => {
 	test('prefers the TLVs when the peer sends them', () => {
 		const dlr = dlrFromPdu(deliverSm(receiptText, {
-			message_state: { tagId: 0x0427, tagValue: 5 },
-			receipted_message_id: { tagId: 0x001E, tagValue: 'from-the-tlv' },
+			message_state: { tagValue: 5 },
+			receipted_message_id: { tagValue: 'from-the-tlv' },
 		}));
 
 		assert.ok(dlr);
diff --git a/test/pdu.test.ts b/test/pdu.test.ts
index d73df4d..f99810b 100644
--- a/test/pdu.test.ts
+++ b/test/pdu.test.ts
@@ -195,22 +195,90 @@ describe('TLVs', () => {
 			},
 			seqNr: 393,
 			tlvs: {
-				5142: { tagId: 5142, tagName: 'Nils', tagValue: Buffer.from('blajfoo', 'ascii') },
-				receipted_message_id: {
-					tagId: 0x001E,
-					tagName: 'receipted_message_id',
-					tagValue: '293f293',
-				},
+				5142: { tagId: 5142, tagValue: Buffer.from('blajfoo', 'ascii') },
+				receipted_message_id: { tagValue: '293f293' },
 			},
 		}));
 
 		assert.equal(pduObj.tlvs.receipted_message_id?.tagValue, '293f293');
-		assert.equal(pduObj.tlvs['5142']?.tagName, undefined);
 
-		const unknown = pduObj.tlvs['5142']?.tagValue;
+		const unknown = pduObj.tlvs['5142'];
 
-		assert.equal(typeof unknown, 'string');
-		assert.equal(Buffer.from(typeof unknown === 'string' ? unknown : '', 'hex').toString('ascii'), 'blajfoo');
+		assert.ok(unknown);
+		assert.equal(unknown.tagName, undefined);
+		assert.deepEqual(unknown.tagValue, Buffer.from('blajfoo', 'ascii'));
+	});
+
+	test('keeps a binary TLV byte for byte through pduToObj and back', () => {
+		const payload = Buffer.from('deadbeef00ff', 'hex');
+		const params = {
+			destination_addr: '46709771337',
+			esm_class: 4,
+			short_message: 'binary payload follows',
+			source_addr: '46701113311',
+		};
+		const parsed = decode(encode({
+			cmdName: 'deliver_sm',
+			params,
+			seqNr: 7,
+			tlvs: { message_payload: { tagValue: payload } },
+		}));
+		const carried = parsed.tlvs.message_payload;
+
+		assert.ok(carried);
+		assert.deepEqual(carried.tagValue, payload);
+
+		const rebuilt = decode(encode({
+			cmdName: 'deliver_sm',
+			params,
+			seqNr: 7,
+			tlvs: { message_payload: { tagValue: carried.tagValue } },
+		}));
+
+		assert.deepEqual(rebuilt.tlvs.message_payload?.tagValue, payload);
+	});
+
+	test('takes the tag id from the record key when the caller gives none', () => {
+		const pduObj = decode(encode({
+			cmdName: 'deliver_sm',
+			params: { destination_addr: '46709771337', short_message: 'hi', source_addr: '46701113311' },
+			seqNr: 11,
+			tlvs: { message_state: { tagValue: 6 }, source_port: { tagValue: 1234 } },
+		}));
+
+		assert.deepEqual(pduObj.tlvs.message_state, { tagId: 0x0427, tagName: 'message_state', tagValue: 6 });
+		assert.deepEqual(pduObj.tlvs.source_port, { tagId: 0x020A, tagName: 'source_port', tagValue: 1234 });
+	});
+
+	test('refuses an unknown tag name rather than putting a wrong tag on the wire', () => {
+		const { buffer, err } = objToPdu({
+			cmdName: 'deliver_sm',
+			params: { destination_addr: '46709771337', short_message: 'hi', source_addr: '46701113311' },
+			tlvs: { nils: { tagValue: 'blajfoo' } },
+		});
+
+		assert.equal(buffer, undefined);
+		assert.ok(err instanceof Error);
+	});
+
+	test('refuses a tag id that does not fit the two octet field', () => {
+		const { err } = objToPdu({
+			cmdName: 'deliver_sm',
+			params: { destination_addr: '46709771337', short_message: 'hi', source_addr: '46701113311' },
+			tlvs: { nils: { tagId: 0x10000, tagValue: 'blajfoo' } },
+		});
+
+		assert.ok(err instanceof Error);
+	});
+
+	test('refuses a TLV too long for the two octet length field', () => {
+		const { err } = objToPdu({
+			cmdName: 'deliver_sm',
+			params: { destination_addr: '46709771337', short_message: 'hi', source_addr: '46701113311' },
+			tlvs: { message_payload: { tagValue: Buffer.alloc(0x10000) } },
+		});
+
+		assert.ok(err instanceof Error);
 	});
 
 	test('round-trips a receipt with message_state and receipted_message_id', () => {
@@ -225,8 +293,8 @@ describe('TLVs', () => {
 			},
 			seqNr: 323,
 			tlvs: {
-				message_state: { tagId: 1063, tagName: 'message_state', tagValue: 2 },
-				receipted_message_id: { tagId: 30, tagName: 'receipted_message_id', tagValue: 450 },
+				message_state: { tagId: 1063, tagValue: 2 },
+				receipted_message_id: { tagId: 30, tagValue: 450 },
 			},
 		}));
 
diff --git a/test/session-extras.test.ts b/test/session-extras.test.ts
index ca48f6f..7501161 100644
--- a/test/session-extras.test.ts
+++ b/test/session-extras.test.ts
@@ -2,11 +2,13 @@ import assert from 'node:assert/strict';
 import net from 'node:net';
 import test, { describe } from 'node:test';
 import type { MessageDlr } from '../src/session.ts';
+import type { PduObject } from '../src/pdu.ts';
 import type { Sms } from '../src/sms.ts';
 import type { SmppServer } from '../src/server.ts';
+import { Reassembler } from '../src/reassembly.ts';
 import { client } from '../src/client.ts';
-import { objToPdu } from '../src/pdu.ts';
 import { server } from '../src/server.ts';
+import { silentLog } from '../src/log.ts';
 
 async function startServer(options: Parameters<typeof server>[0] = {}): Promise<SmppServer> {
 	const { err, server: smpp } = await server({ ...options, port: 0 });
@@ -171,95 +173,77 @@ describe('reconnect', () => {
 });
 
 describe('reassembly bounds', () => {
-	function segment(reference: number, part: number, total: number, seqNr: number): Buffer {
-		const body = Buffer.concat([
-			Buffer.from([0x05, 0x00, 0x03, reference, total, part]),
-			Buffer.from('fragment'),
-		]);
-		const { buffer } = objToPdu({
+	function segment(reference: number, part: number, total: number): PduObject {
+		const udh = Buffer.from([0x05, 0x00, 0x03, reference, total, part]);
+
+		return {
+			cmdId: 0x00000004,
+			cmdLength: 0,
 			cmdName: 'submit_sm',
+			cmdStatus: 'ESME_ROK',
+			cmdStatusId: 0,
 			params: {
 				data_coding: 0,
 				destination_addr: '46709771337',
 				esm_class: 0x40,
-				short_message: body,
-				sm_length: body.length,
+				short_message: Buffer.concat([udh, Buffer.from('fragment')]),
 				source_addr: '46701113311',
 			},
-			seqNr,
-		});
-
-		assert.ok(buffer);
-
-		return buffer;
+			seqNr: part,
+			tlvs: {},
+		};
 	}
 
-	// 0.4.0 held incomplete groups without limit and swept them only when other traffic arrived.
-	test('drops the oldest incomplete message once the cap is reached', async () => {
-		const smpp = await startServer({ maxReassembly: 2, reassemblyTimeout: 60_000 });
+	function collect(
+		reassembler: Reassembler,
+		reference: number,
+		part: number,
+		total: number,
+	): PduObject[] | undefined {
+		return reassembler.collect(segment(reference, part, total), { part, reference, total });
+	}
 
-		let delivered = 0;
+	test('hands back every segment in order once the last one arrives', () => {
+		const reassembler = new Reassembler({ log: silentLog, max: 10, now: () => 0, timeout: 60_000 });
 
-		smpp.on('session', session => session.on('sms', () => { delivered++; }));
+		assert.equal(collect(reassembler, 4, 2, 3), undefined);
+		assert.equal(collect(reassembler, 4, 3, 3), undefined);
 
-		const sock = net.connect({ port: smpp.port }, () => {
-			sock.write(Buffer.from('0000002100000009000000000000002f666f6f0062617200736d70700034000000', 'hex'));
-		});
+		const whole = collect(reassembler, 4, 1, 3);
 
-		let bound = false;
-
-		sock.on('data', () => {
-			if (bound) return;
-
-			bound = true;
-
-			// Three different messages, each only ever sending part 1 of 2.
-			sock.write(segment(1, 1, 2, 10));
-			sock.write(segment(2, 1, 2, 11));
-			sock.write(segment(3, 1, 2, 12));
-			// Completing the first one must not produce a message: it was evicted.
-			sock.write(segment(1, 2, 2, 13));
-		});
-
-		await new Promise(resolve => setTimeout(resolve, 200));
-
-		assert.equal(delivered, 0, 'an evicted message must not be delivered');
-
-		sock.destroy();
-		await smpp.close();
+		assert.ok(whole);
+		assert.deepEqual(whole.map(pduObj => pduObj.seqNr), [1, 2, 3]);
+		assert.equal(reassembler.size, 0);
 	});
 
-	test('expires an incomplete message on its own timer', async () => {
-		const smpp = await startServer({ reassemblyTimeout: 60 });
+	// 0.4.0 held incomplete groups without limit and swept them only when other traffic arrived.
+	test('drops the oldest incomplete message once the cap is reached', () => {
+		const reassembler = new Reassembler({ log: silentLog, max: 2, now: () => 0, timeout: 60_000 });
 
-		let delivered = 0;
+		for (const reference of [1, 2, 3]) {
+			assert.equal(collect(reassembler, reference, 1, 2), undefined);
+		}
 
-		smpp.on('session', session => session.on('sms', () => { delivered++; }));
+		// Completing the first one must not produce a message: it was evicted.
+		assert.equal(collect(reassembler, 1, 2, 2), undefined);
+		assert.equal(reassembler.size, 2);
 
-		const sock = net.connect({ port: smpp.port }, () => {
-			sock.write(Buffer.from('0000002100000009000000000000002f666f6f0062617200736d70700034000000', 'hex'));
-		});
+		reassembler.clear();
+	});
 
-		let bound = false;
+	test('expires an incomplete message once its timeout has passed', () => {
+		let now = 0;
+		const reassembler = new Reassembler({ log: silentLog, max: 10, now: () => now, timeout: 60 });
 
-		sock.on('data', () => {
-			if (bound) return;
+		assert.equal(collect(reassembler, 9, 1, 2), undefined);
 
-			bound = true;
-			sock.write(segment(9, 1, 2, 20));
-		});
-
-		await new Promise(resolve => setTimeout(resolve, 200));
+		now = 61;
 
 		// The other half arrives after the group expired, so it starts a new, still-incomplete one.
-		sock.write(segment(9, 2, 2, 21));
+		assert.equal(collect(reassembler, 9, 2, 2), undefined);
+		assert.equal(reassembler.size, 1);
 
-		await new Promise(resolve => setTimeout(resolve, 100));
-
-		assert.equal(delivered, 0);
-
-		sock.destroy();
-		await smpp.close();
+		reassembler.clear();
 	});
 });
 
diff --git a/test/session.test.ts b/test/session.test.ts
index 05a72a0..87e0cca 100644
--- a/test/session.test.ts
+++ b/test/session.test.ts
@@ -1,12 +1,14 @@
 import assert from 'node:assert/strict';
 import net from 'node:net';
 import test, { describe } from 'node:test';
-import type { PduObject } from '../src/pdu.ts';
+import type { PduObject, PduObjectInput } from '../src/pdu.ts';
 import type { Session } from '../src/session.ts';
 import type { Sms } from '../src/sms.ts';
 import type { SmppServer } from '../src/server.ts';
+import { PduFramer } from '../src/pdu-framer.ts';
 import { client } from '../src/client.ts';
 import { isCommand, objToPdu, pduToObj } from '../src/pdu.ts';
+import { paramText } from '../src/defs/types.ts';
 import { server } from '../src/server.ts';
 
 async function startServer(options: Parameters<typeof server>[0] = {}): Promise<SmppServer> {
@@ -26,28 +28,73 @@ function once<T>(register: (resolve: (value: T) => void) => void): Promise<T> {
 	return new Promise<T>(resolve => { register(resolve); });
 }
 
-/** Binds off a raw socket, which is the only way to declare a version the client cannot. */
-async function bindRaw(smpp: SmppServer, interfaceVersion: number): Promise<PduObject> {
-	const { buffer } = objToPdu({
+type RawPeer = {
+	close: () => void;
+	/** The next PDU the server sends, queued so none is missed between reads. */
+	next: () => Promise<PduObject>;
+	write: (input: PduObjectInput) => void;
+};
+
+/** A peer driven PDU by PDU, which is the only way to say things the client never says. */
+function rawPeer(port: number): RawPeer {
+	const framer = new PduFramer();
+	const queue: PduObject[] = [];
+	const waiting: ((pduObj: PduObject) => void)[] = [];
+	const sock = net.connect({ port });
+
+	sock.on('data', chunk => {
+		framer.push(chunk);
+
+		const { pdus } = framer.next();
+
+		for (const pdu of pdus ?? []) {
+			const { pduObj } = pduToObj(pdu);
+
+			if (!pduObj) continue;
+
+			const next = waiting.shift();
+
+			if (next) next(pduObj);
+			else queue.push(pduObj);
+		}
+	});
+
+	return {
+		close: () => { sock.destroy(); },
+		next: () => {
+			const queued = queue.shift();
+
+			return queued
+				? Promise.resolve(queued)
+				: once<PduObject>(resolve => waiting.push(resolve));
+		},
+		write: input => {
+			const { buffer } = objToPdu(input);
+
+			assert.ok(buffer);
+			sock.write(buffer);
+		},
+	};
+}
+
+function bindOf(interfaceVersion: number, seqNr = 1): PduObjectInput {
+	return {
 		cmdName: 'bind_transceiver',
 		params: { interface_version: interfaceVersion, password: 'pass', system_id: 'user' },
-	});
+		seqNr,
+	};
+}
 
-	assert.ok(buffer);
+async function bindRaw(smpp: SmppServer, interfaceVersion: number): Promise<PduObject> {
+	const peer = rawPeer(smpp.port);
 
-	const sock = net.connect({ port: smpp.port });
-	const response = await once<Buffer>(resolve => {
-		sock.on('connect', () => { sock.write(buffer); });
-		sock.once('data', resolve);
-	});
+	peer.write(bindOf(interfaceVersion));
 
-	sock.destroy();
+	const response = await peer.next();
 
-	const { pduObj } = pduToObj(response);
+	peer.close();
 
-	assert.ok(pduObj);
-
-	return pduObj;
+	return response;
 }
 
 describe('bind', () => {
@@ -161,6 +208,41 @@ describe('bind', () => {
 		await smpp.close();
 	});
 
+	test('advertises the version the server is configured with', async () => {
+		const smpp = await startServer({ interfaceVersion: 0x50 });
+		const asThreeFour = await bindRaw(smpp, 0x34);
+
+		assert.equal(asThreeFour.tlvs.sc_interface_version?.tagValue, 0x50);
+
+		// The threshold for sending optional parameters is 3.4 whatever the server advertises.
+		const asThreeThree = await bindRaw(smpp, 0x33);
+
+		assert.deepEqual(asThreeThree.tlvs, {});
+
+		await smpp.close();
+	});
+
+	test('answers a bind with its own system_id, not the one the ESME sent', async () => {
+		const anonymous = await startServer();
+		const named = await startServer({ systemId: 'the-smsc' });
+
+		assert.equal((await bindRaw(anonymous, 0x34)).params.system_id, '');
+		assert.equal((await bindRaw(named, 0x34)).params.system_id, 'the-smsc');
+
+		await anonymous.close();
+		await named.close();
+	});
+
+	test('answers a refused bind with its own system_id too', async () => {
+		const smpp = await startServer({ authenticate: () => false, systemId: 'the-smsc' });
+		const refused = await bindRaw(smpp, 0x34);
+
+		assert.equal(refused.cmdStatus, 'ESME_RBINDFAIL');
+		assert.equal(refused.params.system_id, 'the-smsc');
+
+		await smpp.close();
+	});
+
 	test('sends no optional parameters to a peer declaring less than 3.4', async () => {
 		const smpp = await startServer();
 		const bound = await bindRaw(smpp, 0x00);
@@ -170,6 +252,23 @@ describe('bind', () => {
 
 		await smpp.close();
 	});
+
+	test('answers a second bind with ESME_RALYBND and its own system_id', async () => {
+		const smpp = await startServer({ systemId: 'the-smsc' });
+		const peer = rawPeer(smpp.port);
+
+		peer.write(bindOf(0x34));
+		await peer.next();
+		peer.write(bindOf(0x34, 2));
+
+		const again = await peer.next();
+
+		assert.equal(again.cmdStatus, 'ESME_RALYBND');
+		assert.equal(again.params.system_id, 'the-smsc');
+
+		peer.close();
+		await smpp.close();
+	});
 });
 
 describe('sending', () => {
@@ -302,8 +401,8 @@ describe('delivery reports', () => {
 
 		assert.ok(session);
 
-		const dlr = once<{ smsId: string; statusMsg: string }>(resolve => {
-			session.on('dlr', resolve);
+		const dlr = once<[{ smsId: string; statusMsg: string }, PduObject]>(resolve => {
+			session.on('dlr', (report, pduObj) => { resolve([report, pduObj]); });
 		});
 
 		const [sms] = await Promise.all([
@@ -319,10 +418,12 @@ describe('delivery reports', () => {
 		assert.ok(sms.dlr);
 		await sms.sendDlr();
 
-		const report = await dlr;
+		const [report, receipt] = await dlr;
 
 		assert.equal(report.smsId, 'dlr-id');
 		assert.equal(report.statusMsg, 'DELIVERED');
+		assert.equal(receipt.tlvs.receipted_message_id?.tagValue, 'dlr-id');
+		assert.equal(receipt.tlvs.message_state?.tagValue, 2);
 
 		session.close();
 		await smpp.close();
@@ -365,6 +466,46 @@ describe('delivery reports', () => {
 		session.close();
 		await smpp.close();
 	});
+
+	test('sends a text-only receipt to a peer that declared less than 3.4', async () => {
+		const smpp = await startServer();
+		const incoming = once<Sms>(resolve => {
+			smpp.on('session', session => session.on('sms', resolve));
+		});
+		const peer = rawPeer(smpp.port);
+
+		peer.write(bindOf(0x33));
+		await peer.next();
+		peer.write({
+			cmdName: 'submit_sm',
+			params: {
+				data_coding: 0,
+				destination_addr: '46709771337',
+				registered_delivery: 1,
+				short_message: 'hi',
+				sm_length: 2,
+				source_addr: '46701113311',
+			},
+			seqNr: 2,
+		});
+
+		const sms = await incoming;
+
+		await sms.sendResp();
+		await peer.next();
+
+		// A raw peer answers no deliver_sm, so this only settles once the session closes.
+		void sms.sendDlr();
+
+		const receipt = await peer.next();
+
+		assert.equal(receipt.cmdName, 'deliver_sm');
+		assert.deepEqual(receipt.tlvs, {});
+		assert.match(paramText(receipt.params.short_message), /stat:DELIVRD/);
+
+		peer.close();
+		await smpp.close();
+	});
 });
 
 describe('a session captured from Kannel', () => {
diff --git a/todo.md b/todo.md
index 06517f0..5ad63d2 100644
--- a/todo.md
+++ b/todo.md
@@ -5,7 +5,7 @@ rules there constrain every item below.
 
 ## Status
 
-The rewrite is **feature complete and green**: 144 tests, lint and typecheck clean, verified on Node
+The rewrite is **feature complete and green**: 155 tests, lint and typecheck clean, verified on Node
 18, 20, 22 and 24. What is left is release work and a few things worth adding before or after 1.0.0.
 
 ```bash
@@ -74,6 +74,11 @@ Every defect listed in the AGENTS.md table has a regression test naming the beha
 - [ ] **In-flight sends across a reconnect.** They currently fail with "Session closed before a
       response arrived" and the caller retries. Re-queueing them automatically would be friendlier
       but risks duplicate delivery, so it needs a decision before it is built.
+- [ ] **`session.ts` is 459 lines.** The one seam left in it is a socket-to-PDU transport, which
+      would move the deliberately public `sock` field out of `Session` or turn it into a getter —
+      a public-surface change, so it waits for a decision.
+- [ ] **`buildPdu` and `dlrFromPdu` carry a complexity of 22.** `eslint.config.js` holds them at
+      that ceiling rather than below the repo-wide 10, so neither can grow but neither shrinks.
 - [ ] **`submit_multi` and the broadcast commands** encode and decode, but nothing exercises them
       end to end. The interop suite is the natural place.
 - [ ] **Move to TypeScript 7** once `typescript-eslint` supports it; `renovate.json` pins TypeScript